In this paper we study the kinetical behaviour of hematite single crystals during their reduction to magnetite. The shape of these crystals was chosen parallelepipedic in order to determine the kinetical laws specific for this case. The reduction rate was found to be under mixed control both by phase-boundary chemical reaction and by gaseous diffusion through the magnetite layer with a relative influence of these two processes depending on reduction temperature. Change in the kinetics of reduction was observed at 800 degrees C.