The chemical compositions of seafloor hydrothermal solutions from the Galapagos Spreading Center, 21 ¿N and 13 ¿N on the East Pacific Rise, the Southern Juan de Fuca Ridge, and the Guaymas Basin, Gulf of California, are examined for systematic variations. Several elemental ratios, elemental products, and empirical geothermometers show much consistency among the various areas, in spite of large absolute differences in elemental concentrations. The results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites. Examination of the solution chemistry in conjuction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry. As potassium appears to be solubility controlled, its use to calculate water/rock ratios is inappropriate. Lithium appears to be a better element to use in these calculations. A model to explain the observed solution chemistries as a result of three-component mixing is discussed. All of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing, a gas-rich vapor phase, and a gas-poor brine phase, or any one of these individual components alone. The vapor and brine phases are the result of phase separation, which may have occurred at some time in the past. Essentially all of the reported data fall within the field defined by these three components. The solutions apparently have had sufficient tine after the mixing event(s) to reequilibrate with the rock. This is true for quartz as well as other chemical components. The best test of this model is chloride. Together with gas and isotropic data, as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18ΔO, and low chlorinity solutions, will have elevated gas contents and lower 18ΔO. ¿ American Geophysical Union 1988