The sintering of zinc oxide spheres has been studied in the temperature range 700 to 900°C in air, oxygen, helium, and hydrogen. Fresh zinc oxide sinters rapidly in air and oxygen, but it does not sinter appreciably in helium in the same temperature range. Zinc oxide spheres annealed separately in air may not be sintered in air at temperatures as high as 1100°C. On the other end, fresh spheres, separately air annealed and subsequently treated in an hydrogen atmosphere, do sinter in the temperature range 700 to 900°C. These facts are explained on the basis of a mechanism involving the diffusional transfer of stoichiometric excess of zinc ions. The diffusion equation for zinc ions is derived, and the experimental points fit the equation satisfactorily. The treatment shows that the relationship between the width of bridges connecting sintering spheres and time varies with extent of sintering. This fact invalidates the use of this relationship as a unequivocal diagnostic criterium for the sintering mechanism in nonstoichiometric oxides.