We're going from Paris to Avignon on a Monday afternoon. Is it okay to purchase the ticket just before boarding (at Gare de Lyon)? Although we get great rates if we pre-purchase non-refundable tickets online, we're not sure if our flights will arrive ontime to make the connection. Has anyone ever encountered problems w/ seat availability when purchasing say 30 minutes to 1 hr before train departure? Thanks.

What time of year are you traveling? In the past, before the PREM's and other discounted tickets were available online, I routinely just showed up at the station and bought tickets, but if it's a holiday, or a school holiday, or the first week in July, or something like that, I wouldn't count on it. I also once missed a TGV I had a reservation for, and they just reissued me a ticket for a later train.

I'm traveling July 12th... which by all accounts appears to fall in the "hectic" time period. Are the PREM tickets also non-exchangeable, i.e. if I miss the train, will I not be able to use it for a later train or go "stand-by" so to speak?

I don't know that July 12th would be that hectic, being a Monday afternoon.

As far as I know, all SNCF discounted tickets carry heavy restrictions regarding refundability, you can't have both big discounts and refunds or everybody would just buy them just in case, I imagine. PREMs are extremely discounted and I think are totally nonrefundable, but the terms are outlined on the SNCF web site.

mc nyc:

I meant to say "first week in August," not July, because that's when hordes of Parisians are heading south. I don't think July 12 would be that bad, and there are quite a number of trains each day.
PREM's are totally unrefundable and unexchangeable. Don't buy one unless you're sure you'll make it to that train.