A series of experiments were conducted to measure the effect of plagioclase composition on the composition of coexisting Cl-dominated hydrothermal fluids. Assemblages containing quartz and natural plagioclase with a composition of either An60 or An82 were reacted with Na-Ca-Cl fluids at 400°C and, 400 bars using flexible cell hydrothermal facilities. Plagioclase remained homogeneous during these experiments despite the fact that homogeneous solid solutions with intermediate compositions are likely metastable with respect to mechanical mixtures at these conditions. It was found that Ca and Na exchange between fluids and minerals appears to be reversible, and highly dependent on plagioclase composition but mCa++ m2Na+ ratios remained constant with increasing Cl for a given plagioclase composition. In fact, a solid solution model which approaches ideal behavior appears to account best for the chemistry of fluids coexisting with An60 and An82 plagioclase. These data may have a direct bearing on hydrothermal alteration processes responsible for the chemistry of hot spring fluids at mid-ocean ridges, which appear to have equilibrated with metastable plagioclase solid solutions of composition An60 to An70.