The activities of the alkali halide salts in aqueous solution are systematically investigated by the molecular simulation of alkali halide salt solutions using a set of ion models developed in previous work by our group in combination with SPC/E water. Five cations and four anions are considered (Li+, Na+, K+, Rb+, and Cs+ and F-, Cl-, Br-, and I-). All 19 combinations of these ions into salts that are soluble in water are investigated (only LiF is insoluble in water). These models were not adjusted to activity data so that the present results are predictions. The OPAS method, which employs a virtual semipermeable membrane, is used to compute the activity data. The predictions are compared to experimental data. Although for salts of the larger ions mostly good predictions are observed, there are important deviations in several of the other systems. A multicriteria approach for including the activity data in the model development is presented.