Initial sintering kinetics of non-stoichiometric CeO2−x

•Initial sintering on CeO2 and CeO2−x was controlled by grain boundary diffusion.•Cation diffusion coefficients were obtained from initial sintering data.•Activation energies of cation diffusion were derived from initial sintering data.•Cation diffusion in CeO2 was caused by a vacancy mechanism. The effect of oxygen non-stoichiometry on the initial sintering behavior of CeO2 was investigated. It was found that the initial sintering of the stoichiometric and hypo-stoichiometric composition was controlled by the grain boundary diffusion. The activation energies of cation diffusion were derived from initial sintering data. Moreover, it is suggested that the cation diffusion was caused by a vacancy mechanism.