Our passports which expire in Aug were just sent in for renewal --expect turnaround in 4-6 wks. May 22nd to June5th.

We're looking at flights to Europe in Sept. that always ask for pp# and ex. date.
Should we use our old pp# and ex. and see what happens, or do we have to wait until our new pp's arrive to book a flight?

You actually don't need a passport to book your international flights but you do need them to fly. I've book scores of international flights on multiple airlines and have never once been asked for my passport info.

If you are booking on line the airline might ask you to put that info in YOUR PROFILE which you can do but again it's not necessary to book the ticket. If you want to enter your old number go ahead and do that - you can simply update your Profile with the new number when you get it.

The only airline that has ever asked me for passport info when booking a flight is Singapore Airlines.

And I just booked flights on the Singapore Air website and was (fortunately) not asked for passport numbers, as mine needs to be renewed before we go.

btw--

I just got my passport renewed. Came last week Priority Mail. It only took two weeks.

mowmow, that's great!