The Seebeck voltage θ and d.c. conductivity σ of sintered polycrystalline specimens of high-purity ferric oxide, made p-type by doping with 0·01-2·0 at.% magnesium, were measured at temperatures up to 1300°C. Carrier concentrations calculated from θ, approximately constant between 450 and 900°C, showed that each Mg²⁺ introduced one p-carrier up to 0·2 at.% Mg; above 900°C θ decreased due to oxygen loss, measured thermogravimetrically. Three temperature regions were distinguished in the non-linear log σ against 1/T plots (T in °K): region A, less than 450°C, where σ was affected by grain growth and porosity; region B, 450-900°C, from which an expression for hole mobilities was derived and where the observed conductivities were fitted by calculated values; region C, greater than 900°C, where n-carriers arising from oxygen loss made an important contribution to σ. Deviations in electrical properties above about 0·2 at.% Mg suggested the formation of magnesium ferrite and the coexistence of the spinel and haematite phases was confirmed by x-ray diffraction and optical examination.