The phase space theory of chemical reaction in which all accessible channels are uniformly weighted was shown to be inappropriate for describing exothermic reactions. Modification to the theory was introduced by a concept of partial statisticity in which only that portion of the phase space available to each vibrational level was treated statistically. The product rotational and translational distributions of the reaction, Cl + HI, and its isotopic variant, Cl + DI, were computed by this modified formulation and compared with that obtained from a straight application of the phase space theory. The implication of the profound differences found from these two formulations was discussed in light of experimental data.