In emulsion polymerization the number and size of the latex particles govern not only the colloidal behavior of the reacting system but also the rate at which conversion of the monomer progresses. Because of the importance of these factors a wide variety of synthetic latexes have been studied with the electron microscope. The changes in the latex particles—from swollen monomer-polymer particles at low conversion to the final latex particles after removal of the unreacted monomer—have been observed for a typical SBR system. The effects of various post polymerization treatments on the size and size distribution of the latex particles have also been observed.