Different surfactant and costabilization systems were studied for the miniemulsion polymerizations of styrene (St) and of butyl methacrylate (BMA). It was found that the combination of sodium dodecyl sulfate, Triton X-405, and octadecyl acrylate yielded excellent results. With this system all of the droplets initially present in the reactor polymerized. This stabilization system was then used in the study of compartmentalization of the monomer droplets in miniemulsion polymerization. Blends of BMA and St miniemulsions were prepared separately and polymerized together to study mass transfer in this system. It was observed that no matter is exchanged between the different phases. This compartmentalization was also demonstrated and exploited by producing a bimodal molecular weight distributions latex, achieved with a semicontinuous process. The lower MWD was created by adding a second miniemulsion containing a transfer agent. (C) 2003 Wiley Periodicals, Inc.