The effect of the reactive surfactant HITENOL BC20 (polyoxyethylene alkylphenyl ether ammonium sulfate; 20 U of ethylene oxide (EO)) on the emulsion polymerization of styrene was studied via reaction calorimetry (Mettler RC1; 70 degrees C). For polymerizations carried out above the cmc of BC20, the reaction kinetics and evolution of the number of particles (N-p) resembled those reported using the conventional surfactant sodium lauryl sulfate, indicating that nucleation proceeded by a combination of micellar and homogenous nucleation (Interval I and Stage 2). The reaction rate (R-p) not only increased with increasing initiator (K2S2O8) and surfactant concentrations, as expected, but the increase in rate in Stage 2 was dependent on the initiator concentration and independent of the surfactant concentration. This is consistent with the proposed nucleation mechanisms. The molecular weight increased with increasing surfactant concentration and decreasing initiator concentration as would also be expected for a conventional surfactant. The dependencies of R-p and N-p on the BC20 and initiator concentrations, however, were lower than the classical Smith-Ewart values (R-p proportional to [E](0.47-0.52); R-p proportional to [I](0.21-0.25) 0.25) although R-p was found to be directly proportional to N-p. Chain transfer to the reactive surfactant is considered a likely source of divergence. (c) 2008 Wiley Periodicals, Inc.