The kinetics of the thermo-oxidation of NR samples was determined by measuring Wallace plasticity (P-t) 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 P-t with ageing time, in accordance with the literature. However, for short durations (t less than or equal to 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.