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The unreinforced 6061 alloy and a 6061/SiC composite, at the beginning of the T6 temper, were submitted to thermal cycling (up to 2000 cycles) in the temperature ranges 25–180 °C and 25–220°C. The microstructure of these materials was studied by scanning electron microscopy and mercury intrusion porosimetry. Mechanical and thermomechanical properties were also investigated. Tensile strength, hardness, Charpy impact resistance, fracture toughness, density, specimen dimensions and thermal expansion were compared before and after thermal cycling. The unreinforced alloy showed an increase of ductility with cycling. This behaviour was chiefly due to overageing and partial annealing. The composite material, following the thermal treatment, did not only undergo overageing and annealing of its metal matrix; void coalescence, crack formation and reinforcement debonding were also observed. The development of these defects basically resulted in a marked decrease of tensile strength, yield strength, Charpy impact energy and fracture toughness.