3 Rb-Sr whole rock isochron ages for Thai granitoids are presented. These new results and a systematic survey of available Rb-Sr whole rock isochron data indicate that there is substantial evidence for granite magmatism in Thailand at c. 240 Ma (Lower Triassic),c. 210 Ma (Upper Triassic),c. 130 Ma (Lower Cretaceous) and c. 90 Ma (Middle Cretaceous). K-Ar ages for mica separates are commonly grossly discordant—up to about 150 Ma younger than the corresponding Rb-Sr whole rock isochron age. This resetting is attributed to water/rock interaction in hydrothermal circulation systems probably driven by deep burial in areas of high heat flow. Hot springs are common in Thailand, with estimated subsurface water temperatures up to 200°C. The chemistry of the Thai granites allows them to be grouped into I-types or S-types (magnetite-series or ilmenite-series). These 2 types of granite are related respectively to subduction of oceanic lithosphere and continent-continent or continent-magmatic arc collisions. The major tin deposits of Thailand are associated with collision related S-type granites of Triassic and Middle Cretaceous age.