The thermal degradation of natural rubber/polystyrene (NR/PS) interpenetrating polymer networks (IPNs) was studied and the effects of blend ratio, crosslink level and initiating system were analysed. The thermogravimetric analysis (TGA) shows that the IPNs are more stable than the pure components. The full-IPNs have better stability than semi-IPNs which is due to higher entanglement density of full-IPNs. The initial decomposition temperature and temperature for 50% decomposition (T50) increase with increase in concentration and crosslinker level of the plastic phase. The kinetics of degradation were analysed using nine mechanistic equations. The kinetic parameters such as activation energy, Arrhenius parameter and entropy of activation were determined. Thermal ageing studies of the samples were also conducted. The IPNs aged for 72 h at 100°C showed enhanced mechanical strength due to crosslinking on post curing.