A series of yttria-doped ceria (YDC)/gamma -alumina supported nickel samples were prepared by co-impregnation and used as catalysts for CO2 methanation. Temperature-programmed reduction analysis was employed to characterize the catalysts. The activity and methane selectivity of these catalysts were measured and compared. It was found that the activity of the nickel catalyst in CO2 methanation could be enhanced significantly by the surface oxygen-vacancies of ceria due to the formation of interfacial Ni-Ce3+ active centers. The activity and selectivity enhancement with yttria content of the YDC-supported nickel catalyst is suggested to result from the participation of intrinsic bulk oxygen vacancies of YDC. With increasing yttria doping into ceria, increasing number of bulk oxygen vacancies can participate steadily in the activation of CO2, thereby leading to the observed enhancement of activity and selectivity. Both CO2 conversion and methane selectivity reached their respective maxima at ca. 10 mole% yttria in YDC. When yttria composition in YDC was greater than 10 mole%, the observed loss of catalytic activity was due to excess yttria, which resulted in segregation of the surface yttria species and covering of the surface oxygen vacancies. Enhanced dispersion of the nickel oxide species, which was responsible for their reduction at lower temperatures, was possibly due to the presence of the yttria-rich surface, thus impeding crystallite growth of the nickel oxide species.