Zinc oxide samples have been prepared by the thermal decomposition of zinc oxalate in oxygen and then sintered by isothermal heat treatment in the temperature range 673–973 K in controlled pressures of nitrogen and water vapour. Sintering was followed by conventional methods of surface area measurement; the samples reached stable minimum values of specific surface area which were characteristic of the treatment temperature and atmosphere pressure. These “equilibrium values” of surface area showed a logarithmic dependence upon the temperature. An explanation of the sintering process has been proposed. In addition, the nitrogen adsorption isotherms obtained at various stages in the sintering of the samples fit the de Boer “standard” isotherm only for those samples which have reached “equilibrium” surface area values above 673 K. At other stages in the sintering the samples exhibit enhanced adsorption. The suggested explanation for this phenomena is that the freshly prepared zinc oxide exists with a high surface energy brought about by its preparation in an oxygen atmosphere. The initial surface has excess of oxygen which is depleted by the diffusion of zinc ions from the bulk to the surface to provide homogeneity.