The mechanical properties of polystyrene-interleaved carbon fibre reinforced epoxy composites, which exhibit controllable stiffness, have been investigated. DMTA and flexural tests showed that the storage modulus and flexural stiffness of these composites could be reduced by up to 98% when heated from 20 °C to 120 °C and the stiffness was fully recoverable on cooling. The flexural stiffness of the interleaved composites at room and elevated temperatures were predicted using simple beam theory and were found to be in good agreement with the measured values. Compressive and tensile properties were significantly reduced at 120 °C due to the presence of the softened polystyrene interleaves. Flexural strength tests at 20 °C indicate that there is a need for improvement of the adhesion between polystyrene and carbon fibre reinforced epoxy plies.