Equations have been derived which model element partitioning between silicate melts, aqueous fluids, and crystalline phases during crystallization. These equations can be used to calculate the efficiency of removal of elements from magmas into aqueous fluids as a function of (1) the bulk solid-liquid partition coefficient of these elements; (2) the initial and saturation water concentrations in the melt (which together determine the amount of melt crystallized before water saturation); and (3) the chlorine concentration of the melt (in the case of chlorine-complexed cations).The efficiency with which copper and molybdenum can be removed from silicate melts, E(Cu) and E(Mo), respectively, has been calculated. Based on geologic data, copper is modeled as a compatible element and molybdenum is modeled as an incompatible element. Under these conditions the ratio E(Mo)/E(Cu) increases as the initial water concentration of the melt decreases for a given depth of vapor evolution and a given Cl/H 2 O ratio and increases as the depth of vapor evolution increases for a given Cl/H 2 O ratio and a given initial water concentration of the melt.Cu is concentrated so efficiently into a moderately to highly saline aqueous phase that liquid-vapor extraction seems to be a reasonable process to account for the concentration of Cu in porphyry Cu deposits. Efficient extraction of Cu results when aqueous fluids are evolved early in the crystallization of the intrusion. The value of D(Mo) is small relative to D(Cu) at moderate to high chloride concentrations, and the extraction of Mo from the melts into aqueous fluids therefore tends to be less efficient. However, vapor-liquid partitioning can extract the requisite quantities of Mo from granitic melts of batholithic size if Mo acts as an incompatible element and if the water content of the magma at water saturation is on the order of several weight percent.