Semibatch emulsion polymerisation of butyl acrylate with neat monomer feed in the presence of sodium lauryl sulphate as emulsifier and potassium persulphate as initiator was investigated. The effects of monomer feed rate, emulsifier concentration, initiator concentration, temperature, and pre-period time on the kinetic features were studied. It was shown that a steady state is established which can be best described by the correlation suggested by Wessling as 1/R-p = 1/K + 1/R-a. An expression was derived for instantaneous monomer conversion which can predict the effects of important variables such as feed rates, seed composition, and seed contents on the evolution of steady state. Those predictions were compatible with the experimental results. For all practical purposes, it was shown that the time it takes to reach the near steady-state increases with monomer feed rate for non-swollen particles, and decreases with monomer feed rate for monomer-swollen particles. However, the time required to reach the real steady state is always minimum at the lowest monomer feed rate for all seed compositions. Pre-period time was shown to have considerable effects on the time required to reach a steady state, but had no appreciable effects on the final number of particles formed or on the steady-state rate of reaction at the conditions studied. However, the particle nucleation process was influenced by the size of the initial monomer charge and by the duration of the pre-period. The average number of radicals per particle reached a steady-state value early in the polymerisation. Polymerisation rate and particle number both increased with temperature.