The batch emulsion polymerization of styrene was studied as a function of the initial monomer charge. The emulsifier (sodium lauryl sulfate) concentration was fixed at a level in excess of its critical micelle concentration (cmc). The mechanism of the polymerization was examined through data obtained on the rate of polymerization (through reaction calorimetry), the evolution of the number of particles (through the particle size distribution), and the free emulsifier in the aqueous phase (through a mass balance and surface tension measurements). At low monomer/water ratios (final solids content Υ
10%) nucleation took place throughout the entire reaction, with micelles existing throughout; the rate of polymerization increased sharply to a maximum and decreased just as sharply with no constant rate period. At the higher monomer levels (final solids content Υ 20%), the increase in rate was initially as sharp as at the lower levels of monomer; however, this was followed by a slower increase in rate brought about by the nucleation of particles in the absence of micelles (homogeneous nucleation); the length of this nucleation increased with increasing monomer charge. A rate maximum was reached at a similar conversion in each experiment, signaling the disappearance of monomer droplets; however, this did not necessarily indicate the cessation of nucleation. All polymerizations proceeded in a similar fashion until the disappearance of the monomer droplets. Based on these and previous results, a modified description of the emulsion polymerization process is proposed. This description rests on the main characteristic of Interval II, which is that it cannot be characteristically represented by a constant rate of reaction and a constant number of particles for emulsifier concentrations above the cmc, contrary to what is generally considered valid; instead it is represented by an increasing rate of polymerization and a continued particle formation. The particles formed during this period, referred to as Stage II to differentiate it from the classical Interval II, are produced by homogeneous nucleation, and the end of this stage is marked by the disappearance of monomer droplets. Particle nucleation may or may not end at this time. In many cases, the shape of the rate of polymerization curve provides by itself the main characteristics of the polymerization process, namely: the end of micellar nucleation, the end of homogeneous nucleation, monomer droplet disappearance, decreasing monomer concentration in the polymer particles, and onset of the gel effect. However, supplementary information, particularly the evolution of the number of particles, is essential to monitor the nucleation process.