The ageing kinetics and mechanisms of a composite solid rocket propellant were investigated by monitoring unstressed propellant samples during prolonged storage at elevated temperatures. For samples confined under air during ageing, it was determined that oxidative cross-linking of the propellant binder was the main degradation mechanism over time. Plasticizer loss was a significant ageing mechanism only for those samples aged unconfined. In addition, there was an indication that ambient humidity had a significant but reversible effect on propellant mechanical properties. Arrhenius mathematical relationships were derived in order to ascertain the extent to which ageing was accelerated by increased propellant temperature. An activation energy for binder oxidation of between 71 and 74 kJ/mol was determined.