Where do we go through customs

If you were going AMS (Amsterdam) to CDG, your wife would be correct. You would (generally) go through customs at your first point of entry to the Schengen Zone.

However, England is not part of the Schengen Zone, and therefore I believe you will actually end up going through customs twice--once in London and once in Paris. No one wins this wager.

I haven't actually done this, so it's possible one of the more experienced travelers on this board will contradict/correct me, but that's my understanding.

It also depends on how the airport is set up. Some airports it is possible to avoid customs because international transit passengers are contained in one area of the airport.

If you stay in the international lounge at Heathrow, you will never "officially" enter the country. You won't go through either passport control or customs. I think that if you change terminals, at least with some of terminals, you will have to go through passport control, but if your luggage is checked through all the way to France, you probably won't have to go through customs until you get to CDG.

I've changed planes at Heathrow once, on the way to Belgium, and I went through neither passport control nor customs. However, it was in the same terminal. Things might have changed.

Check with your airline.

If you were flying to CDG through FRA with checked luggage, you would go through passport control in Frankfurt, when you entered the Schengen zone, but customs in France, but UK is not a part of the Schengen zone. You will go through passport control and customs in Paris regardless.

I forgot to mention we are flying British Airways. Thanks for the info.

You will go through immigration and customs at CDG; the UK is not park of the Schengen agreement. But you will have to go through another security screening, and you they will ask to see your passport. But this is part of the security procedures to verify your identity, and not immigration/customs. But, be carefully of your terminology, a lot of people use the term "customs" incorrectly. Customs deals with the "things" you're bringing to the country. Immigration is the actual passport check. Thus if you were flying into a Schengen country like Amsterdam(instead of LHR), the immigration(passport) check would be at Schipol airport, and the customs(baggage) check would be at CDG.

Michael - Thanks for the info on the terminology it is greatly appreciated.

I just hope my wife doesn't use this as a loop hole to get out of the wager.

That's what I was thinking:)

If you are only changing flights at Heathrow you will not go through customs or immigration, even if you are changing terminals. I fly to Europe from the other direction and have also passed through Singapore, Bangkok and Hong Kong airports without going through customs.

You have to go through security again at Heathrow - not immigration or customs. Here's the info page from British Airways' website.