Newly updated bus times. Ekkamai to Pattaya, and Mo Chit to Pattaya (and vice versa) will be the primary image, and I'll also put Suvarnabhumi to Pattaya and Pattaya to Hua Hin in the comments.
There is a bus every hour from Ekkamai to Pattaya from 6:00-21:00. The specific times shown in the image are buses that you can purchase tickets for ahead of time from the website.
There is also a bus every hour from Pattaya to Ekkamai from 4:30-21:00 and the times shown are buses you can purchase tickets for ahead of time.
Top left is Ekkamai to Pattaya, top right is Pattaya to Ekkamai.
Bottom left is Mo Chit to Pattaya, bottom right is Pattaya to Mo Chit.
Prices are 138₿ for all routes shown in this image.
Reservations can be made www.rrcticket.com
During holidays and other busy times, there will be buses every 20-30 minutes.
Also there are almost always mini-buses (vans) going from Ekkamai to Pattaya and Pattaya to Ekkamai as well. They depart when they have enough passengers to be full.
