When I go on bahn.de it shows (3) train schedules from Paris Nord or Paris Est to Mainz HBF. When I go on the SNCF.com site it only says 'this trip requires more than 3 changes." It also doesn't pull up the same trips that www.bahn.de pulled. Am I doing something wrong?

Me and these darn train sites.

Would I be ok in booking on www.bahn.de once it got closer to my travel time (August)? Or since I'm departing from Paris do I need to book on the SNCF (French) site?

Thanks again with help on one of my train questions.

... and yes, I am leaning towards spending my last night in Europe in Mainz instead of Frankfurt.

I take it that you realise that the trains from Paris Est to Mainz all seem to be night trains with sleeper to Strasbourg, and then changes at Karlsruhe etc to get to Mainz.
The trains from Paris Nord are genrally the Thalys to Bruxelles, and then either a Thays or ICE to Koln, and then another train to Frankfurt and another to Mainz. Neither is a short nor a direct trip. The Thays is highspeed and has compulsory bookings. There are several each day, so reservations a day or so in advance should be possible. You can book in Paris all the way to Mainz on whichever route. I'd wait till I was in Paris before making the booking some days in advance. (In fact, I'll be doing exactly that in September this year to go from Paris to Koln!)

Thank you adeben.
The trip I just pulled up on bahn.de was:

Paris Nord Sa, 17.06.06 dep 6:55

THA 9409 THALYS

Köln Hbf Sa, 17.06.06 arr 10:45

Köln Hbf Sa, 17.06.06 dep 10:53 EC 101 EuroCity

Mainz Hbf Sa, 17.06.06 arr 12:38 5 a/b

Duration: 5:43; runs not every day, 26. Apr until 2. Sep; not 4. Jun, 15., 16. Jul, 19., 20. Aug
Border crossing: Aachen Süd(Gr)

I'm concerned where is reads "runs not every day, 26. Apr until 2. Sep; not 4. Jun, 15., 16. Jul, 19., 20. Aug"

August 19 happens to be the day I would need to take it. Does this mean that this train is not available on August 19th? Oh Noooooooooo.

forgot to mention that the time (train schedule) is different in August than it is June. is this common?

Yes, schedules are seasonal. Some trains only run on certain days of the year. If you are requesting the schedule for the 17th of June, DB might very well show a train that does not run on Aug 19. Specifically request connections for Aug 19 (19.08.06).

French Rail's website is a ticketing website, so they only show connnections for which they sell tickets online. Apparently they do not sell tickets for connections with more than three changes. In most cases, they also do not sell tickets for trips with portions entirely within another country (such as Köln to Mainz). French Rail also doesn't sell tickets, or show connections, for dates more than two months in advance.

You can book the Thalys portion on the Thalys or French Rail website, and the Köln to Mainz ticket separately on the German Rail website. You would pick up the Thalys ticket at a station in France and the DB ticket would be mailed to you. Your best price might be the 8:52 EC from Paris Est to Saarbrucken and the regional train from there to Mainz via Bad Kreuznach at €75,60. You could buy these tickets online at DB and have them mailed to you here.

To see all of the EC connections from Paris, use the DB website and filter out Thalys by selecting "without ICE" under Means of Transport.