The data on ionic activities of metal salts in water are useful in constructing stability diagrams, pourbaix diagrams, the U.S. Bureau of Mines Mineral Technology Program, corrosion abatement, and environmental preservation. A new method is described in detail for determination of aqueous ionic activities, and an equation is proposed for expressing the activity coefficient as a function of concentration. The method consists of highly precise measurement of water vapor pressure by using differential pressure transducers, and a solution calorimeter for determination of enthalpies of solution and heat capacities. Results indicate that the available data for numerous salts of the metals beyond the fourth group have inaccuracies originating from various sources. Experimental data for many binary and multi-component salt solutions are not available. Three methods of estimation are described, and their success is discussed. It is shown that significantly more accurate estimation methods for multicomponent metal salt solutions must await adequate new data.