A model of the liquid-vapor phase partitioning behavior of low concentrations of gas tracers in water at geothermal temperatures and pressures is presented. This model uses Henry's coefficient to describe the variation of the gas tracer solubility with temperature and pressure. A new method is described for the determination and representation of Henry's coefficients. The method uses experimentally determined values of Henry's coefficient and a theoretically predicted value of behavior at the critical point of water to provide data that can be fitted by a semi-empirical correlation. No assumptions regarding ideal behavior are necessary. The semiempirical correlation is a modified version of that presented by Harvey, A. (1996. Semiempirical correlation for Henry's constants over large temperature ranges. American Institute of Chemical Engineers Journal 42(5), 1491-1494) and better accounts for high temperature and non-ideal behavior. Sets of model coefficients are given for a range of possible gas tracers. The resulting phase partitioning model is simple and may be easily implemented in a numerical geothermal simulator. The use and behavior of the model is illustrated by its application to a number of idealised test problems.