Commercial emulsion polymerization processes are often done with high-solids recipes (e.g., about 50 wt % monomer), resulting in relatively viscous, pseudoplastic reaction mixtures. The change in rheological behavior during high-solids emulsion polymerization complicates the operation in terms of imperfect mixing, increasing heat transfer resistance and reactor fouling. In this article, we report the influence of solids content on the colloidal stability of polystyrene (PS) and polyvinyl acetate (PVAc) latex systems. For the systems investigated, solids content up to 50 wt % had no influence on the colloidal stability of the latex. The influence of recipe on colloidal stability is more pronounced than the influence of operating conditions. Brownian coagulation dominates over shear coagulation for both low- and high-solids systems, although in some cases the operating conditions appear to have some effect on the course and outcome of high-solids emulsion polymerization.