The osmotic second virial coefficient is a key parameter in light scattering, protein crystallisation. self-interaction chromatography, and osmometry. The interpretation of the osmotic second virial coefficient depends on the set of independent variables. This commonly includes the independent variables associated with the Kirkwood-Buff, the McMillan-Mayer, and the Lewis-Randall solution theories. In this paper we analyse the osmotic second virial coefficient using a Gibbs-McMillan-Mayer framework which is similar to the McMillan-Mayer framework with the exception that pressure rather than volume is an independent variable. A Taylor expansion is applied to the osmotic pressure of a solution where one of the solutes is a small molecule, a salt for instance, that equilibrates between the two phases. Other solutes are retained. Solvents are small molecules that equilibrate between the two phases. The independent variables of the solvents are temperature, pressure, and chemical potentials. The derivatives in the Gibbs-McMillan-Mayer framework are transformed into derivatives in the Gibbs framework. This offers the possibility for an interpretation and correlation of the osmotic second virial coefficient using activity coefficient models. (C) 2009 Elsevier B.V. All rights reserved