Adsorption of methane on high-surface-area materials, especially zeolites, is a potential means for increasing the on-vehicle fuel storage capacity of vehicles using natural gas. Gravimetric methods have been employed to determine the room temperature adsorption capacities of a number of high-surface-area materials at up to 1,000 psig. On all zeolites tested, except one, the initial rates of adsorption, representing better than 80 percent of the total adsorption, are fast. Two techniques for correcting crude high- pressure adsorption isotherms for nonadsorbed gas have been investigated, and it appears preferable to estimate the free volume using a fluid which cannot penetrate the pores of the zeolite. The potential of zeolites for gas storage is discussed.