The activity coefficients of dilute solutions of oxygen and sulfur in some binary alloys are analyzed in terms of three models—the quasi-chemical theory, the ideal solvent model, and the coordination cluster theory. The first theory does not give a satisfactory representation of a large fraction of the data. The ideal solvent model provides a good representation of a majority of the systems. However, the coordination cluster theory provides a consistently good description of the data. Only one unknown parameter is present in the analytical expressions of the activity coefficients of the solute in the latter two theories. This parameter is related to physical quantities and, in principle, can be independently determined experimentally or theoretically.