This work is an extension of a communication reported by two of the authors [Carro and Herrera-Ordonez, Macromol Rapid Commun 2006, 27, 274], where bimodal particle size distributions (PSD), obtained by asymmetric flow-field flow fractionation (AFFF, AF(4)), were taken as evidence of certain degree of stability of primary particles. Now, emulsion polymerizations of styrene were performed under conditions employed before by other researchers, intending to examine if the behavior observed is general. The number of particles (N) and PSD were studied by means of dynamic light scattering and AF4. By the later, bimodal PSDs were detected in all cases, where the population corresponding to primary particles (diameter <20 nm) depends on reaction conditions. Regarding N, AF(4) results show that it is constant during interval II, in contrast to DLS results. Primary particle coagulation was evidenced as minimums in N evolution and the rate of polymerization curves, monitored by calorimetry and gravimetry, which are enhanced when higher particle number is generated and/or the ionic strength is increased. These results suggest that particle coagulation is not as extensive as it would be expected according to the coagulative theory. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 31523160, 2010