what is the availability of trains between barcelona and monpellier.
times? is there an early train
Cost? should i try to buy tickets here or can we get them in barcelona
Length of trip? (hrs?)

for years the only direct train before afternoon leaves Barcelona about 9am and terminates at Montpellier, where connecting TGV or Corail trains are waiting. There are hourly connections also by changing at the border but since tracks in Spain are wider than those in France most trains physically cannot go thru the border - but this Talgo train has wheels that they can crank apart or together at the border, making thru-running possible. Since there are only two daily Talgos going thru the border they seem to be quite full everytime i've taken them - as the alternative - changing at the border is a hassle and much longer.
How long will you be in Barcelona before the train? And i think the worst could be that you'd have to pay more to ride first class if second class were all sold out. You could try www.renfe.es - the Spanish Railways web site but i am not sure you can book this international train there - in any case folks have lots of frustrations (from posts here) using the renfe.es site. In the U.S. RailEurope (www.raileurope.com) books these trains and actually can have discounted tickets as well - though a $18 mailing fee makes them more expensive. For any REurope product i always advise calling Byron at BETS 800-441-2387; www.budgeteuropetravel.com) - IME you'll get great personal attention and can talk to someone knowledgeable - unlikely at RE IME - but if you will be in Barcelona some days ahead of time i think you will be able to book the train without problem - except IME a lengthy wait in line - it should be somewhat cheaper booking there as well. You can find more about this train also at www.seat61.com i believe. so i guess if your whole trip depends on being on that train try renfe.es first and see if you can get it to work - and ask questions as several Fodorites have been very knowledgeable about getting that fickle site to work and will help you with it. And if you will be somewhere else in Spain beforehand you could book it at any Spanish train station or travel agent - you need not be in Barcelona i think to book it.

There is a direct train at 8.45 am (leaving from Barcelona França station) - arrival Montpellier at 1.22 pm.

Or leave from Barcelona Sants station at 4.42 pm and arrive Montpellier at 9.09 pm - also a direct train.

DH and I took train from Barcelona to Montpellier in 2006 so I don't know how things have changed but this is what we did:

We were in B for a week and as soon as possible, took the blue subway line to Sants and looked for renfe signs

One took a number (ours was 153 and they were on 102 but it went fast) and they didn't wait around for slow pokes either!

It cost 49 euros each. The train left at 8:45 (we traveled on a Wednesday) from Francia station-which is NOT the main station so be sure your driver understands that. Other days, you can get trains from Sants station.

It was interesting to hear announcements go from Spanish and Catalan to French. We understood nothing. It took about 5 hours to reach Montpellier. As luck would have it, DH took a bathroom break with our passports in his safe pocket when the French police came through asking for passports! My fractured French explanation brought smiles. They never did look for my DH! We took the same train to Nice a few days later after making Montpellier our base. We really enjoyed the city.

PS-we did not change trains.

Renfe also has stations in the center of Barcelona - like Franca (France) station, which is where actually the 8:45am Talgo train departs from - Sants is a few miles out so head over to Franca - or RENFE also used to at least have ticket outlets in the heart of Barcelona.

My info has the 8:45am departing from Franca station but it is from last year so i'll check - but usually you can buy those train ducats at any station inside a country.

Don't know if it matters or has changed but 'Francia' was what ticket agent wrote on a piece of paper for us to show our taxi driver.

Do make certain you get the correct station.

Schedules i referenced for tomorrow show the 8:45am Talgo train for Montpellier departing from the Estacion de Franca, in the center of Barcelona, near the waterfront i believe - a gorgeous old wrought-iron train shed that was not used much for years until rehabbed for the Olympics i believe - much nicer than shopping-mall-esque Sants station, a few miles out of town. The Talgo indeed not long ago depart from Sants i believe.

And in a few years when the Pyrenees base rail tunnel is finished there should be proper regular about hourly rail service Barcelona to points in France service. The new TGV or high-speed rail line Perpignan to Barcelona will not only speed up times but negate the hassle of most train links having to change trains at the border as the new high-speed line is being built to French (and most of Europe) gauge - so there can be thru trains France to Barcelona without having to crank wheels apart at border - and since the Barcelona to Madrid and Madrid to Sevilla high-speed lines are also of French or Continental gauge thru trains Seville-Barcelona-France though this will mainly be of use for freight trains since travel times say Paris to Madrid or Seville will still be very long - and there is also a high-speed Continental gauged line being built Madrid to Hendaye, France, with tie ins to the TGV Atlantique line to Paris that way.
Currently only the overnight Talgo hotel trains Paris Austerlitz to either Madrid or Barcelona provide thru rail links Paris to Madrid or Barcelona - using Talgo stock with wheels that crank apart or together at the border.

The tunnel was finished in due time in 2007 !
Due to environmental reasons (the official Spanish explanation) there is a 3 years delay i.e. no TGV between Perpignan and Barcelona won't run until 2012.

Pvoyageuse - so the Tunnel is finished but how about the standard gauged (France and most of Europe) new high-speed tracks Barcelona to the border? Are they in place? Well if you see this thanks for any insights.

I do not see how there can be thru running Barcelona to say Montpellier without an all-new high-speed tracks in Spain.

Palenque :
From Perpignan to Figueres the track is standard gauged (1.435m) but from Figueres to Barcelone it is still the Spanish gauge of 1.668m.
The official reason for the Spanish delay is that they have to build 2 tunnels, one in Girona and one in Barcelona, hence the "environmental concerns".
So we'll probably be able to take a fast train to Figueres next year (once they have removed the cobwebs in the Le Perthus tunnel and will have to change trains in Figueres......
Incidentally the company which was supposed to take charge is getting compensation.