The decomposition of CO2 over the Cu component of two ZnO/Al2O3 supported Cu catalysts, having different Cu areas, has been studied over the temperature range 393-513 K. The time dependence of the evolution of CO from a CO2/He stream (10% CO2, 101 kPa) which was dosed continuously over the catalyst showed two peak maxima, the first of which moved to shorter times on raising the temperature. The activation energy for the decomposition of CO2 on the ZnO/Al2O3 supported polycrystalline copper was obtained from a plot of the logarithm of the time to the peak maximum of the first peak against the reciprocal of the dosing temperature. The value so obtained was 83 +/- 10 kJ mol(-1) (catalyst A) and 86 +/- 10 kJ mol(-1) (catalyst B) for fresh catalysts reduced in H-2 at 513 K. This value fell to 49 +/- 4 kJ mol(-1) (catalyst A) and 55 +/- 5 kJ mol-1 (catalyst B) after CO reduction at 473 K of the Cu which had been oxidised by the decomposition of the CO2. This lowering of the activation energy for the second CO2 decomposition is considered to be due to the original morphology of the Cu not being restored by reduction in CO after the oxygen-driven reconstruction of the Cu deriving from the decomposition of the CO2.