We consider a new approach to the statistical mechanics of chemical association, A + B ->_<- AB. In principle it is exact, and its thermodynamic basis goes back to Gibbs, but its statistical mechanical implementation in terms of molecular models does not appear to have been given before. For practical computations the success of the method hinges on our ability to calculate the free energy for a mixture of A, B, and AB species, where the species concentrations are regarded as independent. We illustrate the method by analysis of some simple limiting cases.