Tin mineralization in central Thailand is associated with granitic rocks of the Thai-Burmese border range (western group). Granitic intrusions east of Bangkok and near the Kampuchea border (eastern group) have no tin. Fractional crystallization is the fundamental petrogenetic process that controls the evolution of both groups of granites. The tin-bearing alkali-feldspar aplogranites of the western group display an extreme degree of differentiation that has no petrological equivalent in the eastern group. These aplogranites are the product of a combination of magmatic fractionation (primary tin enrichment trend) modified by fluid interaction. The latter process is responsible for a secondary geochemical tin deficiency that is balanced by redistribution of tin in fracture systems—i.e., tin mineralization. The pattern of tin depletion in the aplogranites provides an indication of the tin potential in a given ore-forming system.