The structure and dynamics of methane in hydrated potassium montmorillonite clay have been studied under conditions encountered in sedimentary basin and compared to those of hydrated sodium rnorurnorillonite clay using computer simulation techniques. The simulated systems contain two molecular layers of water and followed gradients of 150 bar km(-1) and 30 K km(-1) up to a maximum burial depth of 6 km. Methane particle is coordinated to about 19 oxygen atoms, with 6 of these coming from the clay surface oxygen. Potassium ions tend to move away from the center towards the clay surface, in contrast to the behavior observed with the hydrated sodium form. The clay surface affinity for methane was found to be higher in the hydrated K-form. Methane diffusion in the two-layer hydrated K-montmorillonite increases from 0.39 x 10(-9) m(2)s(-1) at 280 K to 3.27 x 10(-9) m(2)s(-1) at 460 K compared to 0.36 x 10(-9) m(2)s(-1) at 280 K to 4.26 x 10(-9) m(2)s(-1) at 460 K in Na-montmorillonite hydrate. The distributions Of the potassium ions were found to vary in the hydrates when compared to those of sodium form. Water molecules were also found to be very mobile in the potassium Clay hydrates compared to sodium clay hydrates. (C) 2004 Elsevier Inc. All rights reserved. I