Thermal analysis and the ageing studies of butyl rubber (IIR) compounds have been carried out in order to assess the effect of a silane coupling agent, Bis-(3-triethoxysilylpropyl)tetra sulphide (Si-69) on thermal and thermoxidative stability of IIR. The thermal stability of the raw rubber remained uneffected on adding Si-69, whereas the thermoxidative stability was en ha need. Similar behaviour, though to a lesser extent, has been observed for com pounded rubber. The ageing properties and service life are also improved with Si-69. A suitable mechanism has been proposed.