A rigorous, compact, somewhat unconventional, thermodynamic analysis is presented for the treatment of very precise data on the solubilities of gases in liquids. Relationships among the chemical potentials, fugacities, fugacity coefficients, activity coefficients, molar volumes, and mole fractions, and the standard states involved, are carefully delineated. Both the symmetrical and asymmetrical choices for the activity coefficients are discussed, together with the connections between them. The symmetrical activity coefficient at infinite dilution, γ 2 ° (T,p), is shown to be a very useful parameter for generalizing the ideal solubility concept of Hildebrand and Scott, and to be the factor which links corresponding concepts in the symmetrical and asymmetrical standard states. Arguments are presented for adopting $$\mathop {\lim }\limits_{{\text{as }}x_2 \to 0} {\text{ [}}f_2^{\text{L}} (T,p,x_2 )/x_2 {\text{] = }}k_2 (T,p)$$