Emulsion polymerization of styrene and methyl methacrylate in the presence of a ferrofluid was briefly studied. Thermal properties of the resulted latex particles were investigated by TG–DTA analysis. Determination of the residue weight after the thermal analysis that indicated complete decomposition of the organic components was found to be a facile and practical method to determine the magnetite content in the latex particles. The method was applied to magnetic polystyrene latex particles prepared in the presence of various amounts of the ferrofluid. Analysis of the results suggested that the magnetic content in the latex particles is primarily determined by the weight ratio of the ferrofluid to monomer. © 1993 John Wiley & Sons, Inc.