Emulsion copolymerizations of styrene and n-butyl acrylate were conducted at 70 degrees C using varying amounts of Triton X-405 (octyl phenoxy polyethoxyethanol) as emulsifier. The kinetic behavior was found to vary widely depending on the emulsifier concentration. Unimodal particle size distributions were produced at the lowest (4.2 mM) and the highest levels (12.5 mM, 16.2 mM) of emulsifier while at intermediate levels (6.3 mM and 8.4 mM) bimodal distributions were produced; these were reflected in the reaction kinetics. These results were attributed to the surfactant partitioning behavior in the system which led to homogeneous/coagulative nucleation at the lowest level to homogeneous/coagulative nucleation followed by micellar nucleation at the intermediate levels to micellar nucleation at the highest levels. Although the added surfactant levels were all well above that required to exceed its CMC in the aqueous phase, the substantial partitioning into the oil phase lead to conditions well below the CMC of the portion of the surfactant present in the aqueous phase for the lowest and the intermediate levels of the Triton X-405. Consequences of this partitioning were seen in a semibatch reaction where all the surfactant was initially present in the aqueous phase; in this case, too many particle were nucleated leading to massive coagulation due to insufficient surfactant being available to stabilize the particles.