The Stock Exchange of Thailand main index dropped 0.12%, or 2.04 points, to 1,697.61 on Monday.
Total trading value was 39.7 billion baht, with 8 billion shares changed hands.
SET50 index rose 0.11%, or 1.16 points, to 1,090.40 in trade worth 20.9 billion baht.
MAI gained 0.12%, or 0.63 points, to 545.33 in transactions worth 2.7 billion baht.
Institutional investors were net buyers of 981.4 million baht. Brokers sold 480.7 million baht, followed by small investors (325.5 million baht) and foreigners (175.5 million baht).
Year-to-date, institutional investors were net buyers of 76.7billion baht, followed by brokers (10.9 billion). Individual investors sold 68.6 billion baht, followed by foreigners (19 billion baht).