The influence of oxygen on the kinetics of chemically initiated seeded emulsion homopolymerisation of styrene and the seeded emulsion copolymerisation of styrene/butyl acrylate was investigated by reaction calorimetry. A decrease of the inhibition period and an increase of the polymerisation rate have been observed when the system is purged with nitrogen. It has also been seen that oxygen has a great influence on the polymerisation kinetics, producing not only inhibition, but also decreasing the reaction rate. An increase of the dependence of the average number of radicals per particle with the initiator concentration with respect to theoretical predictions was observed in the presence of oxygen. The overall experimental results show that, after an inhibition period, there is a continuous flow of oxygen from the gas phase, which causes the retardation of the polymerisation rate.