Semibatch microemulsion polymerization of butyl acrylate in the presence of sodium dodecyl sulfate/Aerosol OT surfactant mixture and potassium peroxodisulfate as initiator is analyzed with respect to particle nucleation and growth. It is concluded that particle nucleation occurs during the whole batch and semibatch polymerization periods also in the absence of micelles. The observed increase in the polymer content compared to the batch microemulsion polymerization is mainly due to nucleation of new particles during the semibatch stage. During the post-addition polymerization period particle growth is dominated by particle coalescence. The particular method of semibatch microemulsion polymerization allows to obtain latexes with particle sizes below 40 nm (dynamic light scattering intensity weighted diameter) at polymer concentrations near to 30 wt % and stabilizer amounts between 5 and 6% relative to the polymer mass.