By simultaneously solving the governing equations for defect formation and the relative conservation laws, we have made a general calculation of equilibrium concentrations of point defects in MgO as functions of temperatures, oxygen fugacity (fO2), and Fe content. The aim of this work is to better understand the mechanisms that control the transport processes of electrical conduction and diffusion and the oxidation state of Fe. The present model suggests the following regimes that are consistent with several kinds of experiments. Electrical conduction is dominated at low fO2 by electrons and O vacancies and at high fO2 by Fe3+ on Mg sites (Fe*Mg) and Mg vacancies. Defect associates involving Fe*Mg and Mg vacancies are required to explain experimental data on oxidation state of Fe as a function of fO2, temperature, and Fe content. At high temperature, intrinsic formation of Mg and O vacancies are important controls on Mg diffusivity; Fe3+ impurities at intermediate temperatures and impurity-vacancy associates at low temperatures are important. Associates involving Mg and O vacancies at low temperatures and unassociated O vacancies at higher temperatures are important for O diffusivity. The concentration of Fe at which the concentrations of other defects are affected strongly depends on fO2 and temperature. Even minute amounts of Fe (i.e., 1 ppm) may affect material properties at temperatures below 1273 K and fO2 above 10-15 MPa, whereas at temperatures above 1873 K and fO2 below 10-14 MPa, material properties may be relatively insensitive up to 1% Fe content. ¿ American Geophysical Union 1991