Our passports which expire in Aug. were just mailed in for renewal (expect turnaround May 22nd to June 5th.)

Looking at flights to Europe in Sept. that always ask for passport # and expiration date.
Should we put in our old pp # that expires in Aug and see what happens or do we have to wait until our new pp arrive a month from now to book?

I'll also post this on the air forum, but I'm so used to answers from the Europe Fodorites that I rely on for good information.

I have booked several flights without my passport info and just added it later.

I would not put in my old one as it won't be the same and if I had to guess, that would cause more problems than leaving it blank. good question, though, and I don't know what is proper, you could ask the airline.

Where I have been asked this I have always been able to alter it afterwards through the manage your booking function of the website...

I don't think I've ever added passport info upon booking, I always do it later. You'll be OK without it, just go in later and add it to the reservation.

You can add the new number later. I have my passport number in my FF profile (which information is used to populate the ticket) and will need to get it corrected when I get a new passport. I think the number is only important on the day of travel.

You don't have to have a passport at the time of booking. The airlines needs that information only within 72 hours of your flight.