Hematite-to-magnetite reduction kinetics were investigated mainly in co-co2 atmospheres at temperatures ranging from 500 deg to 1,000 deg c. Reduction rates were measured with a thermobalance, which recorded the weight loss as the specimen was reduced. Reduction data on both synthetic and natural hematite spheres showed good linearity of ro [1-(1-r)1/3] with time, t, up to over 85 pct conversion to magnetite; the rate coefficient (kl, cm/min) obtained from the linear portion of the plot was used to characterize the penetration rate of the hematite-magnetite interface. Gas composition, porosity, and silica content were major factors affecting the rate. Temperature dependency of the reaction rate was small, yielding a mean activation energy of about 3 kcal/mole. Silica additions increased the reaction rate, probably through an increase in the effective surface area and diffusion; however, above 900 deg c, silica can react to form fayalite. Experimental data showed that the linear thickening rate of the magnetite layer, normally evidence of a chemically controlled reaction, can also be indicative of a reaction controlled by both mass transfer and diffusion. Evidence from this study indicates that the dominant rate-controlling factor in the reduction of hematite to magnetite at high temperatures is transport in the gas phase.