Foreign citizens, e.g. pensioners from Europe, cannot import moving goods into Thailand duty-free, even if they have a Non Immigrant O visa. However, personal items such as clothing and books are duty-free.
Original text: http://tinyurl.com/2cxrj36x
Note: Foreigners traveling into the Kingdom in the category Non-Immigrant O who comes to spend his last years or follows a Thai wife is not included in the criteria.
The duty is calculated based on the stated value of the goods being moved or the value estimated by customs (whichever is higher) and is charged at 30% plus 7% VAT.