The diffusive line broadening of the Mössbauer spectrum of Fe1–xO has been studied at 1074 and 1173 K and leads to activation energies for the jump process of 140 ± 20 kJ mol–1 for Fe0·940O and 135 ± 20kJ mol–1 for Fe0·910O. The observed reduction in the average cation-jump frequency from 42 MHz to 30 MHz over the range Fe0·940O to Fe0·910O at 1074 K has been compared with the number of atoms statistically able to jump in random and clustered defect configurations, but the observed behaviour can only be simulated by the incorporation of an activation energy of jumping which increases with increasing cluster size. The absolute magnitude of the derived self-diffusion coefficient was in general agreement with values obtained from radioactive tracer studies.