General equations for modeling open and closed system fluid/rock interaction using trace elements and isotopes are presented. TAYLOR's (1977) open system equation for stable isotopes is extended to account for a change in the ∆ value between rock and fluid during interaction. It is shown that hydrothermal systems in which fractionation factors vary during fluid/rock interaction will significantly alter the isotopic ratios of rocks compared to equivalent systems with invariant fractionation factors. The equations for modeling trace element exchange are analogous to the stable isotope equations and permit the calculation of fluid/rock ratios using changes in trace element concentrations in the rock. Similarly to stable isotope exchange, the change of the trace element concentration in a rock during infiltration is a function of the trace element concentration in the fluid, the fluid/rock partition coefficient and the fluid/rock ratio rather than simply the concentration of the trace element in the fluid.