Based on the copolymerization data of St/MA and St/BA in emulsion and microemulsion polymerization, the mechanism of microemulsion polymerization was discussed. The different copolymerization behaviours between these two systems were attributed to the different monomer ratios in polymerization loci. It was found that for both monomer pairs, the copolymerization behaviour in emulsion system can be well described with the semiempirical equations presented by Maxwell. Due to the much higher content of emulsifier in microemulsion polymerization, copolymerization behaviour in microemulsion is obviously different from that in emulsion copolymerization. The result of St/BA copolymerization strongly hints that the real copolymerization locus in microemulsion polymerization is the interside of the emulsifier layer.