Ayutthaya to Pattaya
 

I want to go from Ayutthaya directly to Pattaya. Do you know how I can go that ? Thanks.

I am sure it can be done, everything is doable in the Kingdom.
I know you can catch a train from the apt to Ayutthaya, takes 30 minutes.
Also know you can get a transfer ranging from minibus to car with driver from the apt to Pattaya. If i were you, and you are not enlightened here, I would contact one of the apt transfer companies and ask for a quote on the transfer. Secondly I would get a quote from an Ayutthaya cabs who will take you half way and then they swap you over with a Pattaya cab coming the other way. Yes they do amazing things and I often wonder what a first time traveller must think when the cab stops in the middle of nowhere, on the side of the road opposite another.
Have a great holiday, yan19454, sorry i don't have more precise info for you.

Taxi? Can't imagine the trip costing much less than 2000 baht though, and finding one at the Ayutthaya end in the first place will be a problem.

I'd just take a bus to Bangkok's Mor Chit terminal (~1.5 hrs) and transfer to a bus to Pattaya (~2 hours), which will cost <200 baht.

I'm just reinforcing what jpatokal said--use the buses. From both places to Bangkok-Mor Chit, buses run about every half hour. That's a really long distance for a taxi unless you don't care about the cost. The buses are pretty comfortable, too.

Yeah of course just take the bus, changing in Bangkok half way to Pattaya.

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cheers.