In this work near-infrared spectroscopy is used to monitor semicontinuous styrene/butyl acrylate emulsion copolymerization reactions. A set of nine reactions with slightly different formulations were carried out. The results of five reactions were used to fit a SIMPLS model, and the four remaining reactions were monitored in order to evaluate the model performance for estimation of important variables and latex properties such as individual monomer concentrations and the average polymer particle diameter. These variables were used to calculate the online monomer conversion, copolymer composition, particle number, and average number of radicals per polymer particle.