Have had this happen on several occasions, and each time I have gotten credit on both airlines. I have always made sure that I never make any misrepresentations in the letters I send to the carriers. I tell the original booked flight that I was diverted off of their flight, and am a member of their FF program, and would really like to get their miles. I then tell the airline I was bumped onto that I was rescheduled to their flight, enjoyed it, and would like to get frequent flyer mileage for the flights I have actually taken. Both airlines have always given me the mileage with no problem. As I see it, the original airline is compensating you for the hassle of the rebooking (and trying to keep your loyalty), and the airline you fly on is trying to win your loyalty (and actually received the money for the flight, and therefore ought to provide you the same FF mileage that everyone else on the flight is getting). To answer the question that I am sure some will ask, I am not aware of any case law on the subject. I think that number of miles involved are too small on any one flight for the companies and the individuals to be litigating over.