We have investigated the effects of valence states and Co-distributions on the coercivity of particulate and thin film Co-treated γ-Fe 2O 3. A maximum Hc of 2 kOe has been obtained in thin films prepared by a new simple method. We found that Co-ions exist in the divalent states probably in the form of ferromagnetic CoFe 2O 4 and that simple CoO to γ-Fe 2O 3 interfaces do not appear to enhance Hc. On the other hand, we observed complex diffused interfaces in a thin film sample with enhanced Hc. The diffused interfaces may be responsible for the Hc-enhancement in Co-modified γ-Fe 2O 3 recording media.