I work for a Foundation that recently used an overseas contractor for some IT services. The contractor submitted an invoice and our bookkeeper said that we have to make a deduction of 15% (?) which has to be paid to the Thai tax department. I've never heard of this before. Does anyone have any information on this? Thanks

quite correct.

Thai companies are required to make a withold tax deduction of this % for all remittences to foreign entities. I'm surprised if any person in business here doesn't know this.