Cylindrical compacts of magnetite were isothermally reduced at 773-1273 K with pure H-2 or H-2-H2O mixtures. The initial reduction rates increased with temperature and partial pressures of H-2 in the H-2-H2O mixtures. However, with progressing reduction, a dense iron layer formed around the wustite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51-89%, depending on the temperature and H-2 partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k(1)(mixed)/k(2)(mixed), k(1)(mixed), k(2)(mixed): contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H-2, H-2-10%H2O, 773 K//H-2-10, 20%H2O, 873 K//H-2-20%H2O, 973 K) the retardation was absent because of the formation of a microporous iron layer product. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.