The dimerization of hard spheres in a hard sphere solvent is studied. Earlier, Cummings and Stell examined this system using the usual Ornstein-Zernike formalism and found that the law of mass action is not satisfied. Here we apply the theory of Wertheim for association and the associative Percus-Yevick closure. Many of our results are qualitatively similar to those of Cummings and Stell. The association constant increases with the reactant density. We find that the reaction equilibria can be controlled by varying solvent density, but to a much lower extent than predicted from the usual Percus-Yevick approximation.
