If one flies internationally to Swampy and then change airlines (separate booking) for a domestic flight does one go through immigration at Swampy?

You would cllear immigration at Suvarnabhumi. 

However there are some airlines that offer a connecting flight arrangement that allows a person to clear immigration at another airport that has allows international arrivals.

You would cllear immigration at Suvarnabhumi. 
However there are some airlines that offer a connecting flight arrangement that allows a person to clear immigration at another airport that has allows international arrivals.

Flight is BA to Swampy and then THAI to Phuket. Flights were booked separately.

I last did it in February. There is a list of possible destinations on the overhead signs directing passengers to transfer points. I think that Phuket is on the list. I believe that it depends on the airport having an international facility. Immigration is a single desk, but I have never seen a queue at it. You will have to clear Customs at the destination.

16 hours ago, ubonjoe said:

You would cllear immigration at Suvarnabhumi. 

However there are some airlines that offer a connecting flight arrangement that allows a person to clear immigration at another airport that has allows international arrivals.

 

Just to clarify, that works out-bound, but not IME in-bound.

 

I flew-in on Qatar end-May, with onward (on the same ticket) to Chiang-Mai on Bangkok Air, and the two-or-three Immigration-desks were just after the transit-to-domestic airline-desks, at Swampy.

 

Only my hold-luggage stayed international until CNX, where I collected it & cleared Customs.

 

 

But in the OP's case, changing airlines & on different bookings/PNRs, he will definitely need to clear Immigration & Customs at Swampy, before checking-in again upstairs for the onward domestic-flight.

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20 hours ago, SABloke said:

If one flies internationally to Swampy and then change airlines (separate booking) for a domestic flight does one go through immigration at Swampy?

Yes.

No way to arrive and enter a domestic flight without immigration check.

 

20 hours ago, SABloke said:

Flight is BA to Swampy and then THAI to Phuket. Flights were booked separately.

Separately booked also sounds that the baggage will not be checked through(?).

So completely standard procedure:

pass immigration

pick up baggage

check-in on 4th floor

pass domestic security check and proceed to gate

 

How much time do you have between the flights?

Just now, KhunBENQ said:

Yes.

No way to arrive and enter a domestic flight without immigration check.

 

Separately booked also sounds that the baggage will not be checked through(?).

So completely standard procedure:

pass immigration

pick up baggage

check-in on 4th floor

pass domestic security check and proceed to gate

 

How much time do you have between the flights?

Thanks for that.

Re: time...I don't know, it's my sister, BIL and their 2 little boys coming. I think they'll be alright (I noticed that there is priority immigration check for families with little kids so hope that speeds up the usual 45min+ process)[