Originally Posted by
mike435
That's an interesting question, I am pretty sure that you have to use same one that you entered with, when you are exiting at the immigration. But in theory you can check in with another one. I don't know if immigration officer would see this or make fuss about it. Would be interesting to hear if someone knows for sure.
For example, I entered Vietnam on my UK passport (no visa fee) and then flew from there (via HND) to the US so had to show my US passport at check-in. I ended up showing both to the immigration officer (as I was stamped out on my British passport) and it was fine. Not sure about Thailand. I suppose I can just use UK for Thailand as there's no difference.