Natural rubber (NR) undergoes chemical changes on heat and air ageing. These changes affect its physical properties and as such, affect the service life of the rubber compound. In this study, a vulcanized NR compound of a typical engine mount composition was subjected to thermooxidative ageing at temperatures from 70 to 110 degrees C, to assess the effect on the tensile properties. The kinetics of degradation of the rubber compound, in terms of changes in these properties, was investigated. A fractional rate law was used to describe the kinetics of ageing in terms of its effect on modulus. Rates of ageing, in terms of effect on modulus, passed through a minimum at about 80 degrees C, indicating the danger of trying to extrapolate in-service ageing behavior from high temperature ageing data. The activation energy of ageing in terms of its effect on modulus, determined for temperatures of 90-110 degrees C, was 151 kJ mol(-1). A second order rate law was used to describe the kinetics of ageing in terms of its effect on tensile strength and elongation at break, with activation energies of 88.32 and 74.3 kJ mol(-1), respectively. According to Ahagon's (Ahagon et al., Rubber Chem Technol, 1990, 63, 683) classification of ageing mechanisms, Type I and Type III ageing mechanisms were predominant. (c) 2006 Wiley Periodicals, Inc.