The microscopic image analysis has been applied for determination of porosity of hematite grains reduced to magnetite at 450 and 850 degrees C, At 450 degrees C, two kinds of voids were analysed : cracks and micropores inside subgrains created by crackling of the initial grain. At 850 degrees C, porosity profiles across the magnetite layers were visible. Also a temporal dependence of the local porosity has been observed. The porosity determined by mercury porosimeter agrees reasonably with microscopic data.