Systematic density-functional calculations have been performed to address an important issue for CO oxidation on redox ceria: the role of lattice oxygen. One major findings is that CO easily grasps one lattice oxygen atom to form CO2- and CO2 on CeO2 (111) and Ce0.75Zr0.25O2 (111) with small activation energies. Zr dopants facilitate the reduction of Ce-4(+) to Ce-3(+) and hence weaken the Ce-O bonds, which benefit the direct formation and release of CO2.