The kinetics of the thermo-oxidation of NR samples was determined by measuring Wallace plasticity (Pt) for different ageing times (t) in a Plasticity Retention Index (PRI) oven at 140 degrees C. For long durations, first order kinetic fully described the reduction in Pt with ageing time, in accordance with the literature. However, for short durations (t < or = 30 min), there was a linear relation between plasticity and ageing time, reflecting zero order kinetics. This showed that PRI depends not only on a thermo-oxidation rate constant, but also on initial plasticity, and that two NR samples with the same PRI do not necessarily thermo-oxidize at the same rate for times of less than 30 min.