The perturbed hard fluid model, which separates solute-solvent interactions into repulsive hard sphere and mean field attractive contributions, is applied to predict solvent effects on the thermodynamics of diatomic dissociation reactions. Theoretically predicted changes in excess Gibbs free energy (Delta G), entropy (Delta S), enthalpy (Delta H), and volume (Delta V) for the dissociation of a homonuclear diatomic dissolved in a monatomic solvent, with Lennard-Jones solute atom-solvent atom and solvent-solvent interaction potentials, are compared with computer simulation results. The perturbed hard fluid model requires only one adjustable parameter, which is determined using simulation results at a single temperature and density. This parameter is used in the prediction of reaction thermodynamics over the entire vapor, liquid, and supercritical fluid regime. Furthermore, the thermodynamics of other reactions, in which the solute atom-solvent atom attractive well depth changes upon dissociation, can be predicted by including one additional parameter, determined using only simulation results for a system with no well depth change.