Shares in coal mining firms fell, with the biggest Banpu Pcl falling as much as 1.7 percent to 594 baht, the lowest since Feb. 1, following Indonesia's plans to impose export taxes on coal and base metals.
Lanna Resources Pcl dropped 1.8 percent to 27.75 baht.
Broker Finansia Syrus Securities said the news weighed on sentiment in the coal miners, including Banpu which owns 65 percent of Indonesian coal miner Indo Tambangraya Megah .
The broader market fell 0.69 percent to 1,190.15, amid weakness elsewhere in Asia due to concerns about euro zone debt problems.
1221 (0521 GMT) (Reporting by Viparat Jantraprap; Editing by Martin Petty)