The kinetics of reduction with hydrogen of thin, dense strips of NiO in the 230-452 degrees C temperature range were measured by the thermogravimetric technique. Pure hydrogen gas at 1 atm was used as the reducing agent. The Fractional conversion-time curves were sigmoid-shaped, indicative of the autocatalytic nature of the reduction process, As relatively thin (54 mu m thick) specimens were employed, the pore-diffusion of gases offered only negligible resistance. The topochemical-reaction-rate constant k(c), deduced from the measured rates using nominal surface area, is found to be given bywhere k(c) is in cm.s(-1). The activation energy for the reduction process appears to be 23.2 (+/-0.65) kcal.mole(-1) in the 230-452 degrees C range.