Dissociation rates of methane hydrate and the rate of transformation of methane hydrate into ice Ih were measured using time-resolved, energy-dispersive X-ray diffraction at atmospheric pressure and temperatures between 148 and 253 K. The dissociation of CH4 hydrate had an initially fast regime, which lasted several tens of minutes, followed by slower dissociation. For temperatures from 168 to 198 K, our analysis indicates that the second step was relatively slow because the CH4 had to diffuse through the thickening ice layer. The resulting diffusion coefficients gave an activation energy of 20.1 kJ/mol and equaled 8.7 x 10(-11) m(2) s(-1) at 198 K. (C) 2002 Elsevier Science B.V. All rights reserved.