Simultaneous measurements of variations in magnetic and dilatometric properties were successfully used to detect the phase transition occurring during the thermal treatment of iron(III) oxide microcrystals. The increase of the compacting pressure, i.e. the increase of the interparticular number, favours the irreversibleγ→α transition. An unusual phase of iron(III) oxide, different from theγ-phase, has been unambiguously demonstrated. Its stability seems to be improved by the surface energy excess due to the highly-divided state.