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 solution theory. On the macroscopic level an expansion of the osmotic pressure is employed. A common statistical interpretation of the osmotic second virial coefficient of the expansion employs the McMillan-Mayer framework and the potential of mean force to characterise the solute-solute interaction. Supplementary to the statistical interpretation. it may be advantageous to develop the McMillan-Mayer framework in a classical thermodynamic context for which we develop the relationship between the state function of the McMillan-Mayer framework and the Helmholtz state function. A Taylor expansion method can be applied to the osmotic pressure of a solution which is dilute with respect to at least one solute component. This offers the possibility for an interpretation and correlation of the osmotic second virial coefficient utilising a McMillan-Mayer state function derived from a Helmholtz state function. (C) 2008 Elsevier B.V. All rights reserved.