I am flying into CDG at 11am and want to take the TGV to Brussels. I have looked into thalys and sncf web sites. Thalys site sucks big time. With the sncf I am only getting an option of a librys fare of Euro 71. Will it make sense to not book anything now and buy a ticket when I get to CDG? There are bound to be many vacant seats bewteen Paris and BXL.

Is this one way or round-trip?

If one-way the fare will probably not change at all.

If round-trip you should buy now.

If you are planning to travel over a holiday period don't be so sure that there will be "many" seats available. I have been on trains between Brussels and CDG that were completely sold out.
If you are flying over a nonholiday period (and I mean holidays in the Netherlands, France and Belgium), yes, you should have no trouble getting a ticket when you will arrive. But you are unlikely to find a seat at the very lowest price--those sell out fairly quickly.

I guess the cheapest way, even if you're traveling one way, is to buy the cheapest return and throw away the return. But as far as I know, the cheapest return will not be much cheaper than the one way (Thalys has the fares online).

So I think that if you're traveling one way, it's probably ok to wait, with BT's caveats in mind. In any case, the one-way ticket is completely refundable and changeable anyway (well, I guess 90% refundable or something like that if you miss the train).

Thanks for your replies. This is a one way and I think I will wait till I get there. I don't think there is a holiday period in the first week of April there
Maybe I will just get to Lille first and then go to Brussels.

I looked up the prices for you:

http://www.thalys.com/fr/en/tarifs?s...p;d=15-04-2005

(I used a random date of 4/15.)

Librys Mezzo Mezzo+ Smilys
71.50 107 79 49

Note that the last three numbers are for the round-trip and the first is Librys one-way.

Unless you can get the Smilys round-trip at 49 euros, stick with the Librys one-way at 71.50, and I guess it should be ok to buy the ticket when you get there. The Mezzo fare will not really help you.

Another thing you can do is to check for last-minute fares on thalys.com -- they're posted on the website, and you can get lucky.

By the way, I checked the prices for CDG to Brussels. Presumably if you go through Lille the comparison will be similar.