The electrical conductivity of Cr2O3 nominally doped with 2 mol% MgO has been studied by the four point a.c. technique as a function of the oxygen activity (O-2 + Ar, CO + CO2 and H-2 + H2O) in the temperature range 400-1200 degrees C. It is concluded that Cr2O3 doped with MgO is an extrinsic conductor and that the dissolved ME-dopant is compensated by the formation of electron holes at near atmospheric oxygen pressures and by oxygen vacancies (or possibly interstitial chromium ions) at highly reduced oxygen activities (in CO + CO2 and H-2 + H2O gas mixtures). In H-2 + H2O mixtures Mg-doped chromia also dissolves hydrogen as protons and significantly affects the defect structure and electrical conductivity. The defect structure of the oxide under various conditions is discussed.