Each ticket in Carnet expires within 24 hours?

Sonia, go to ratp.fr, click on 'English(flag)', 'Discover Paris', then on 'T-ticket'. That's far more concise and complete than I could ever say!

UNUSED TICKETS are good for basically forever (see the bottom of the above-mentioned page). Otherwise, tickets are good for only 90 minutes FOR TRANSFERS between bus/bus, bus/tram, and tram/tram. Apparently, you can still ride the metro ALL DAY on a single ticket...if you wanted to ;-).........Be Sure and Understand the different types of transfers you may and may not make with t-tickets (AKA 'metro tickets'). You CAN transfer between metro/metro, metro/RER, and RER/RER WITHIN Paris. I would just buy a carnet. You can buy them as you need them - either the packs or individual t-tickets. BUT many stations have done away with manned counters...you'll need coins/Euros for the machines. The larger stations will be manned. Or womanned.

Sonia, two good websites that may help you are parisbytrain.com and tomsguidetoparis.com

A carnet is nothing really,(not like a booklet or pass) , it is just they will hand you ten single tickets at a slight discount then buying them one by one. Each one is used for one bus ride,, NO transfers,, or a metro ride anywhere, WITH TRANSFER ability. Buy one carnet at first,, see how much you use it,, you may find you walk alot more then you think, I find Paris very compact,, so one carnet,, and perhaps a few singles more is all you may need . There is no time limit on tickets that are not used.

The old "t" ticket was not good for bus transfers. The new "t+" ticket includes bus-bus transfers or bus-tram transfers up to 90 minutes after boarding the first bus or tram. A single "t+" ticket is good anywhere the metro goes (some lines end in zone 3) and on the RER in zone 1 only (central Paris). A ticket is good for unlimited transfers until you exit the system. There is a one-day pass called Mobilis that costs €6.10 for zones 1-2. It is good only on the day you first use it, not for 24 hours. Unless you are going to take six or more rides on a single day, using tickets from a carnet will be cheaper.

We had a few tickets left from a carnet that we wouldn't use, and left them for hotel housekeeping.

Sonia, If you intend to return to Paris on your next trip to France, it doesn't matter whether you buy a carnet and have left over tickets since they do not expire and stay valid. You will be prepared for the next trip since you already have valid Metro tickets in your possession. I inquired about this on this last trip in June so that I'll have Metro tickets already once I come back. No need to deal with the machine then or with a clerk. In June a carnet cost 11.60 Euro. Get it now because you're going back!!

We actually had some carnet tickets this year from 2007 that were not accepted, so I can't say that they don't ever expire.

Janis, their website says they don't ever expire - but you need to take non-working (and never-used) tickets to the counter and exchange them for new ones...