Particle formation is a key step in emulsion polymerization reactions and has been the subject of extensive investigations in the past few decades. The main aim of this work was to investigate, both theoretically and experimentally, the conditions for secondary nucleation and particle evolution in batch and semi-batch emulsion polymerization. The effects of variation in monomer and emulsifier concentration in the feed, the distribution between the charge and the feed, temperature and the emulsion feed rate on polystyrene particle size distribution were investigated both theoretically and experimentally. The population balance and kinetic models developed were employed for predicting the product attributes for a range of reactor operating conditions. The sets of nonlinear algebraic and integro-differential evolution equations were solved efficiently for this work.Monomer and surfactant feed rates were found to have significant effects on the growth of polymer particles and consequently on the particle size. Different particle sizes and distributions were obtained using the same procedure with variable operating mode. A semi-batch reactor with variable monomer emulsion feed can produce latexes with variable polydispersity. A high initial rate of particle formation could lead to reduction in secondary nucleation and hence to the formation of a mono-modal PSD. This can be achieved by using high initiator and emulsifier concentrations in the feed, a high temperature, or a low monomer concentration in the charge. A low initial rate of nucleation increases the possibility of secondary nucleation and the formation of a bimodal PSD. The evolution of a bimodal PSD requires secondary nucleation after primary nucleation occurs.