Isothermal measurements of the dehydroxylation of kaolinite at temperatures from 400-540°C and with controlled water vapor pressures from <10-3 mm Hg to 175 mm Hg are shown to conform with a single curve of α (fraction reacted) versus t/t50, where hn is the time for t50 percent reaction. The curve agrees well with the supposition that the reaction is diffusion controlled rather than obeys first-order kinetics. The reaction rate constant depends on both the temperature and the ambient water vapor pressure. The activation energy, 51 kcal/mole at <10-3 mm Hg, increases with vapor pressure, becoming 112 kcal/mole at 47 mm Hg. The rate constant diminishes with vapor pressure according to an equation of the type log (1 – kp/ko) = m+n logP, where m and n are temperature-dependent parameters, which is interpreted in terms of a partial coverage of the dehydroxy la ting surface by chemisorbed water molecules. The criteria for distinguishing first order kinetics from diffusion controlled reactions are reviewed critically.