Bus to Siena

The R131 is the rapid bus between Florence and Siena

I found it helpful to go to the bus station and purchase the ticket. We requested a bus schedule to confirm what time to be there.

We went to the Florence bus station to buy tickets in advance (not necessary but we were in the area and just wanted to save a little time on the day we made the bus trip) and we looked at the paper schedule posted at the bus station, it shows which buses are Rapido. They go once an hour, I think. You could also ask driver to confirm, "Questo autobus, e Rapido?"

Thank you all for the great information and I will stop by the bus station and get the tickets the day before we need them.
Was wondering if I could get the return tickets at the bus station in Florence as well?

Yes, ask for "andata e ritorno" tickets. I think the round trip cost for two of us was about 30 euros. Validate these when you get on the bus.

The bus station is just around the corner from the train station...literally a two minute walk. When you arrive by train, just walk over and ask some questions, pick up a schedule and buy your tickets... this is the easy and convenient way to make Sienna a day trip. There is also a convenience shop nearby if you want to pick up a few items.

you can also buy them at any Tabac. Definitely have the return trip ticket in hand. They are not timed tickets; once you have the ticket you can take any bus to or from Siena. When we arrive in Siena we make careful note of the times for return and especially the second to last bus which is generally early evening -- that way if we misjudge there is still one more bus. They run 2 or 3 times an hour and it doesn't make that much difference between the rapid and regular bus -- if it is easy to connect with the rapid fine, but the difference is slight.