Is it possible to leave Thailand by air on the same day that a 30-day visa exempt entry stamp expires - e.g. valid until July 30 and leave on July 30 - then return a day later for another non-exempt entry?

 

I know there's no limits to entry by air, but should there be a day or two in between returning back to Thailand? For example, stay one night or two nights in Cambodia then return to Thailand.

No problem with that.

There is no absolute prohibitions on back-to-back visa exempt entries by air. However, you should be aware that there is a vague instruction to immigration that use of visa exempt entries to facilitate long term stay in Thailand should not be allowed, with that policy being applied flexibly. Some officials apply this quite strictly. Most likely, it will not be a problem if you have fewer than six visa exempt entries total since 2015, and have spent only a few months in Thailand in the last year. If you have spent the bulk of the last year in Thailand (with a mix of tourist visas and visa exempt entries) I would counsel against trying back-to-back visa exempt entries, even with a couple of days out of Thailand between them.

4 hours ago, RandolphGB said:

I know there's no limits to entry by air, but should there be a day or two in between returning back to Thailand? For example, stay one night or two nights in Cambodia then return to Thailand.

There is no published limit, but there is effectively a limit that is decided by the border IO’s.

 

If you are doing back to back visa exempt entries, and staying for most/all of the 30 days, you should expect to get warned or stopped at some point. It’s the time spent in the country rather than the number of entries that they’re most interested in.