The stability of binary mixtures of chemically dissimilar, monodispersed spherical particles has been investigated using the light scattering technique recently introduced by Lips and Willis (29). Time dependent scattering intensity curves were interpreted in terms of the Smoluchowski theory of coagulation. The system to be reported consisted of polyvinyl chloride (PVC) latex and hydrous chromium (III) oxide in aqueous solutions of simple urhydrolyzable electrolytes. This chosen pair of colloids consisted of spherical particles of narrow size distribution and afforded the opportunity to vary the surface potential of one component (chromium hydroxide) while maintaining an almost constant potential on the other (PVC). The variables investigated were the zeta potential, the relative number concentration, and the size of the primary particles. Most experiments were conducted at a constant ionic strength below the critical coagulation concentration, thus preventing rapid electrolytic destabilization. In this paper the experimentally determined stability ratio will be compared with theoretical calculations based upon recent theories of heterocoagulation. It will be seen that there exists excellent qualitative agreement between experiment and theory for mixtures of these two colloids. Good quantitative correlation is difficult and a few explanations of the discrepancies will be offered.