The effect of temperature, strain-rate and ageing on the crack growth mechanism in a composite propellant has been examined to obtain an understanding of the fracture process under service life conditions. Both as-received and aged specimens were tested at each of three strain-rates and temperatures. The materials were aged by subjecting them to various thermal loads (accelerated ageing, thermal cycle and thermal shock) designed to expose them to conditions similar to that experienced by a rocket motor during its service life. The fracture behaviour of the propellant specimens were affected by changes in temperature whilst strain-rate had only a marginal effect over the range studied. It was found that as the material temperature decreased from 60 to - 40 degrees C the stiffening of the propellant caused increased hysteresis ratios and decreased crack velocities. The deterioration of the mechanical properties of the propellant depended on the severity of the thermal loads. In each case the accelerated aged specimens became harder and more brittle whilst the thermally cycled and thermally shocked specimens were only marginally affected. A distinct difference in the mechanism of crack growth was observed for the accelerated aged specimens, along with a marked decrease in hysteresis ratio, critical stress and critical strain and an increase in crack velocity.