The electrical conductivity of Cr2O3 has been studied by the four point ac technique as a function of oxygen activity (O2 + Ar, CO+CO2 and H-2+H2O mixtures) in the temperature range 440-1500-degrees-C. This paper focuses on the conductivity behaviour at temperatures below about 1000-degrees-C. Under these conditions it is concluded that the oxide exhibits extrinsic behaviour determined by the presence of dissolved divalent cation (Mg) impurities and accordingly the oxide is a p-conductor. In O2 + Ar and CO + CO2 mixtures the conductivity exhibits a small oxygen pressure dependence (approximately sigma is-proportional-to P(O2)1/20), and this is interpreted to reflect slow equilibration kinetics and a decreasing solubility of the cation impurities with decreasing oxygen activity. In H-2 + H2O mixtures equilibration kinetics are relatively rapid and the conductivity is dependent on both the oxygen and hydrogen activity; this demonstrates that hydrogen dissolves in chromia and significantly affects the defect structure and electrical conductivity. The defect structure of chromia is discussed.