A general purpose computer program especially adapted to the study of acid-base and coordinative interactions and dis-solution and precipitation in aqueous systems has been de-veloped. The program uses the stability constant approach and the Newton-Raphson methodfor digital computation of equilibria. It is able to handle numerous species and to find the equilibrium set of solids. Gas phases are considered to be at constant partial pressures. The method is explained by use of the aqueous iron (III)-calcium-carbonate-phosphate system as an example. An application of the program to a system of 788 soluble species, 83 possible solids, and one gas-phase com-ponent is given as an illustration.