Emulsion polymerizations of styrene were performed with different emulsifier and initiator concentrations, using an automated reaction calorimeter. The reactions were compared to the classical description of the emulsion polymerization process in terms of the shape of the rate of polymerization curves and other characteristic events such as the nucleation process. For reactions performed above the critical micelle concentration (cmc) of sodium lauryl sulfate, the absence of the constant rate period (Interval II) was evident. In all these cases, the heat of reaction (rate of polymerization) increased up to-a maximum and then decreased. This maximum was determined to primarily indicate the disappearance of the monomer as a separate phase (i.e., droplets). For experiments carried out below the cmc, the kinetics and evolution of the number of particles can be described by the classical qualitative description of emulsion polymerization proposed by Harkins.