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 CO2. The CO2 peak is a result of the reaction between NiO and adsorbed CO. The CO2 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 degrees C), with two overlapped CO peaks, whose temperatures are independent of the catalyst composition, and no peak for CO2. (The two overlapped CO peaks are attributed to CO molecularly adsorbed on Ni.) (b) A high temperature range (400-700 degrees C), with peaks for both CO and CO2, 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 CO2 via the reaction 2CO --> 2C + 2O --> CO2 + C and the reduction of some of the NiO that remained unreduced in H-2. The CO2 generated is adsorbed on MgO sites. The other fraction of CO dissociatively adsorbed desorbs as CO.