The effect of the addition of gypsum to a solution of calcite in water (or the addition of calcite to a solution of gypsum in water) is determined theoretically for arbitrary temperature, carbon dioxide content, and degree of saturation with respect to calcite (or gypsum). The results can be used to study the chemical evolution of waters flowing through alternating gypsum and limestone strata, to predict conditions which will lead to the precipitation of calcite and/or gypsum, and to predict the chemical evolution of evaporating calcite–gypsum solutions. It is shown that under some circumstances, simultaneous gypsum (or calcite) precipitation and calcite (or gypsum) solution may occur. Coprecipitation of gypsum and calcite is found to be unavoidable if a solution is evaporated to dryness, although the initial precipitate will be a single mineral. The methods used and many of the conclusions drawn are applicable to other systems of two or more minerals.