The rates of oxidation of CO to CO 2 by particles of a co-precipitated mixture of CuO and Al 2 O 3 (355–500μm) have been measured in a fluidised bed; these measurements were not affected by interphase mass transfer or the deposition of carbon. Efforts were made to minimise sampling problems by using a rapid-response mass spectrometer to analyse the gases from the hotter beds or those fluidised by gases with high [CO]. However, the base-line of the mass spectrometer drifted with time, so an infra-red analyser measured [CO 2 ] and [CO] in the off-gases from cooler beds, in which CO was oxidised to CO 2 more slowly. These techniques enabled rates of reaction to be measured from 250 to 900°C; the order of reaction in CO was always close to unity. It was established that above 500°C, the oxidation of CO by particles of CuO was controlled mainly by external mass transfer. At ∼250°C, it appears that CO reacts directly with solid CuO in: (1)CuO+CO→Cu+CO2.However, above ∼700°C, CuO particles seem to react by the shrinking core mechanism, predominantly involving the two consecutive steps:(3)2CuO+CO→Cu2O+CO2,(4)Cu2O+CO→2Cu+CO2.The activation energies and pre-exponential factors for reactions 1 and 4 were measured.