Emulsion polymerization of methyl methacrylate was carried out using a mixture of anionic emulsifier, sodium dodecyl sulfate (SDS) and nonionic emulsifier, poly(oxy ethylene) nonyl phenyl ether with an average of 40 oxy ethylene units per molecule (NP-40). The rate of polymerization and the number of polymer particles produced both increased with increasing the amount of NP-40 initially charged, when NP-40 was used as the sole emulsifier. Interestingly, however, both the rate of polymerization and the number of polymer particles produced was found to decrease, when the amount of NP-40 added to the fixed amount of SDS is increased in the mixed emulsifier. On the contrary, they both increased when the amount of SDS added to the fixed amount of NP-40 is increased in the mixed emulsifier. The mixed emulsifier did not affect the number of the radicals per polymer particle at a certain number of polymer particles, but affected the number of polymer particles, thus the rate of polymerization. (c) 2005 Elsevier Ltd. All rights reserved.