The unseeded semicontinuous emulsion copolymerization of methyl methacrylate and n-butyl acrylate using mixed surfactant systems (anionic SLS (sodium lauryl sulfate) and non-ionic Brij35) in a wide range of concentrations and at different SLS/Brij35 ratios (1/0, 1/1, 1/3, 1/9, 0/1) was studied. Two different kinetic behaviours led to reactor ’starved’ conditions with single anionic or with mixed surfactant systems above a critical concentration, and reactor ’flooded’ conditions with single non-ionic or with mixed surfactant systems at the lowest concentration. The use of mixed surfactant systems, compared to the use of SLS alone, led to a competitive effect between decrease in particle number due to the addition of Brij35 and increase in particle number due to the increase of the total concentration of emulsifier. Moreover, narrower particle-size distributions (PSDs) with larger average particle sizes were obtained with mixed surfactant systems than those obtained with single anionic systems. The PSDs at the early stages of reaction lost the positive skewness obtained in soapless emulsion copolymerization with the addition of emulsifier, SLS being more effective than Brij35. High total concentration of emulsifier led to broad PSDs.