The temperature-programmed desorption (TPD) of CO adsorbed on unreduced and reduced NiO/MgO solid solution catalysts was investigated. For the unreduced NiO/MgO, two peaks were observed: one for CO and another one for CO 2 . The CO 2 peak is a result of the reaction between NiO and adsorbed CO. The CO 2 thus generated is adsorbed on the MgO sites, whereas CO is adsorbed on the Ni generated through the reduction of NiO by CO. For the H 2 reduced NiO/MgO, two temperature ranges for CO TPD were identified: (a) A low temperature range (50–200°C), with two overlapped CO peaks, whose temperatures are independent of the catalyst composition, and no peak for CO 2 . (The two overlapped CO peaks are attributed to CO molecularly adsorbed on Ni.) (b) A high temperature range (400–700°C), with peaks for both CO and CO 2 , whose temperatures depend on the composition of the catalyst. They are attributed to CO species dissociatively adsorbed on Ni. A fraction of these dissociatively adsorbed species of CO are converted to CO 2 via the reaction 2CO→2C+2O→CO 2 +C and the reduction of some of the NiO that remained unreduced in H 2 . The CO 2 generated is adsorbed on MgO sites. The other fraction of CO dissociatively adsorbed desorbs as CO.