On gamma-Al2O3 and delta,theta-Al2O3, the adsorption of CO at approximately 300 K (and occasionally at approximately 77 K) has been investigated by IR spectroscopy and adsorption microcalorimetry. Up to three types of sigma-coordinated CO adspecies form, two of which are ascribed to sites (most likely tetrahedrally coordinated Al ions) in crystallographically defective configurations, and one to sites (still most likely tetrahedrally coordinated Al ions) located on regular crystal planes; the relative amounts of the three CO adspecies depend on the alumina crystal phase, and on the temperature of vacuum activation. The overall amounts of CO adsorbed are quite small, as compared to the extent of surface dehydration : surface reconstruction and ion shielding effects are supposed to be involved in the activation process. The heat of adsorption of the three adspecies, extrapolated to zero coverage, correlates nicely with the corresponding CO stretching frequencies, but at higher CO coverages the observed heats fall to very low values, incompatible with the sigma-coordination mechanism proposed and with the observed frequencies. The occurrence of reversed surface mobility processes, brought about by the adsorption phenomenon and involving an endothermic contribution to the net heat release observed, is postulated.