Heterogeneous latexes were prepared by a two-stage seeded emulsion polymerization process at 80 degrees C using potassium persulfate as the initiator and sodium dodecyl sulfate as the emulsifier. Poly(styrene-co-methacrylic acid) latexes containing varying amounts of methacrylic acid (MAA) were used as seeds. The second-stage polymer was poly(isoprene-co-styrene-co-methacrylic acid). By using different methods for the addition of the MAA and by varying the amount of MAA, the hydrophilicity of the polymer phases could be controlled. The morphologies and size distributions of the latex particles were examined by transmission electron microscopy. The latexes were in all cases unimodal, and had narrow particle size distributions. The particles displayed different morphologies depending on the polymerization conditions and monomer composition. The hydrophilic properties of the two phases in combination with the internal particle viscosity and crosslinking of the second phase during polymerization were found to be the major factors influencing the particle morphology.