Styrene miniemulsions were polymerized at 70 degrees C in an automated reaction calorimeter with samples being periodically removed and analyzed for their particle size distributions by both transmission electron microscopy (TEM) and capillary hydrodynamic fractionation (CHDF). It was shown that the average diameters obtained using CHDF and TEM agreed to within 5%, while the standard deviations obtained agreed to within 14%. Using CHDF, the evolution of the particle size distribution during miniemulsion polymerizations was monitored. This technique showed the increase in the average particle size with increasing conversion, as expected, and also an increase in the standard deviation with increasing conversion. These results are consistent with those obtained using TEM.