Since there seems to be some confusion/conflict in a recent thread about this topic, I thought with all your help we could nail down the exact requirements that people wishing to volunteer in Thailand should demand from the organisation they are (typically) paying to place them.

If someone wants to volunteer legally in Thailand, what documents/permits/visas do they need?

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Owner, travelforum.org
My latest travels: Shikoku Pilgrimage

Required for any kind of work, paid or unpaid, is a workpermit.

With a non-immigrant visa, with exemption of retirement, one can apply for a workpermit. Application for a workpermit has to be done from within Thailand, application for the visa at embassy/consulate outside of Thailand.

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Diving Phuket and Similans

This ThaiVisa Forum has useful information on visas.
Thailand Forum

There are some tourists that use an ordinary 30 day Tourist Visa Exemption when they do volunteer jobs. Often these jobs are for a week or two but they are technically in violation of their entry permission. Do most of them who only want to work a week or two at an orphanage, wash elephants or change diapers from Aids patients get caught - probably not!
Visa Frequently Asked Questions

Eleven or twelve years ago I did several volunteer jobs with Earthwatch; two of them in Thailand involving archeology in the Phimai area. I used a regular free 30 day visa exemption entry. This was a "volunteer" job where I paid for lodging and support of the project plus worked at a dig. In my country money used for a legitimate charity can be a tax deduction, the money I paid for the project and the plane ride over to Thailand was a tax deduction. (Of course that was before retirement and back when I actually had a job where the deductions actually helped me tax wise!)
Earthwatch: Origins of Angkor - Earthwatch

A lot might also depend on how long you intend to volunteer at one of these places. If this volunteer duty is for a short time, without pay, out of a longer holiday you might get away with it. If you are working for a legitimate charity concern for longer than a week or two I think they should be taking care of your work permits. Just an opinion.

Agree with Koonel.

In some cases the risk of getting caught is very, very small, but the workpermit is officially required in all cases of (volunteer) work.

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Diving Phuket and Similans

Thank you very much for clarifying this, and for the extra info and links - I will add it to the FAQ. Hopefully it will bubble up in Google and will point those looking to volunteer in the right direction when ensuring they are in Thailand legally.

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Owner, travelforum.org
My latest travels: Shikoku Pilgrimage