Self-diffusion coefficients of oxygen in both polycrystalline and single crystal 2MgO·TiO 2 have been measured at an ambient oxygen pressure of about 40 mm Hg over the temperature range 1080–1450°C. A convenient method to estimate the volume diffusion coefficient, D 1, of this polycrystal in which the relative magnitude in D g (grain boundary diffusion coefficient) with respect to D 1 falls between two extremes, i.e., D g = D 1 and D g ⪢ D 1, is newly proposed, and its plausibility is examined by comparing the resultant D 1 with that of single crystal 2MgO·TiO 2. Not only the magnitude but also the temperature dependence of the “apparent” diffusivity of these polycrystalline particles considerably varied with their particle size. The reason for this semiquantitatively interpreted in terms of the relative magnitude of D g with respect to D 1.