The Stock Exchange of Thailand main index fell 0.25%, or 4.35 points, to 1,702.17 on Tuesday.
Total trading value was 52.3 billion baht, with 8.4 billion shares changed hands.
SET50 index advanced 0.06%, or 0.68 points, to 1,101.36 in trade worth 27.5 billion baht.
But MAI dropped 0.05%, or 0.29 points, to 538.57 in transactions worth 2.8 billion baht.
Institutional investors were net buyers of 1 billion baht, followed by brokers (544.9 million baht). Individual investors sold 847.7 million baht, followed by foreigners (706.4 million baht).
Year-to-date, institutional investors were net buyers of 84.2 billion baht, followed by brokers (11.6 billion). Individual investors sold 70.9 billion baht, followed by foreigners (25 billion baht).