Correlation of laboratory and field observations of mineral assemblages in quartz-cassiterite veins and adjacent greisen alteration zones in the Southeast Asian tin belt with equilibrium constraints generated from thermodynamic calculations can be used to assess the composition of the hydrothermal solutions responsible for the transport and deposition of tin. The hydrothermal mineral assemblage in the Southeast Asian tin deposits consists of quartz, cassiterite, muscovite, K-feldspar, topaz, magnetite, and rarely, hematite, fluorite, tourmaline, and zinnwaldite. Depth estimates and fluid inclusion homogenization and freezing temperatures indicate that this assemblage formed at approximately 350 degrees C and approximately 500 bars in the presence of an electrolyte solution containing approximately 1 molal (m) total chloride. Thermodynamic calculations indicate that the solution pH, fugacity of oxygen, fugacity of H 2 S, activity of K (super +) , activity of Ca (super +2) , and the total molality of F (super -) were approximately 4.5, approximately 10 (super -31) , < or =10 (super -1) , approximately 0.1, approximately 10 (super -3) , and approximately 10 (super -2.1) , respectively, during the mineralizing process. Tin was apparently transported in solution primarily as stannous chloride and hydroxide complexes. The presence of topaz precludes significant transport of tin as fluoride complexes. Solubility calculations indicate a minimum total tin concentration in solution on the order of 10 ppm. The intimate association of cassiterite and the greisenized wall rock adjoining the veins suggests that tin mineralization occurred in response to increasing solution pH accompanying hydrothermal alteration of the host rock.