Dispersion of liquid-liquid systems is commonly applied in industrial processes such as extraction, suspension, and emulsion polymerization. This article describes the influence of the quality of emulsification on the course and outcome of a batch-emulsion polymerization of styrene and vinyl acetate. From visualization experiments and polymerizations in combination with reaction calorimetric studies, a critical impeller speed, N*, can accurately be determined for a particular reactor setup and a given recipe. The results show that styrene-water emulsions are more difficult to emulsify than vinyl acetate-water emulsions. In general, a large turbine impeller appears to be more effective in emulsifying monomer-water dispersions than a pitched-blade impeller. In addition, for vinyl acetate emulsion polymerization, the possibility of premixing the reaction mixture has been investigated.