The effects of oxygen in the emulsion polymerization of styrene have been studied by varying the initial dissolved oxygen concentration in the aqueous phase. Using different initial levels of dissolved oxygen corresponding to 0, 50, 80, 90 and 100% of saturation, the induction period, conversion kinetics, molecular weight and particle size were investigated. The length of the induction period did not vary linearly with the initial oxygen level, suggesting diffusion from the headspace to the aqueous phase was an important factor. Partitioning calculations and experimental data suggest oxygen should be modeled as both a water-soluble and monomer-soluble inhibitor. (C) 2000 Elsevier Science Ltd. All rights reserved.