Aluminosilicate nanotubes (imogolite) have been synthesized from highly concentrated inorganic solutions by hydrothermal treatment. These can be converted to microporous nanofibers with a pore radius in the range of 0.3-0.6 nm referring to the results from the nitrogen adsorption isotherm. The water vapor adsorption isotherms indicated that the natural imogolite plotted a proportional isothermal curve where the amount of adsorbed water increased in proportion to P/P0: the maximum amount of adsorbed water was ca. 60 wt%. The synthetic imogolite showed a rapid increase at 0.9-0.95 range of P/P0 and achieved a maximum of ca. 80 wt%, with a better methane storage property than that of the usual compressed natural gas storage. In order to obtain a high ratio of water adsorption and a large methane storage capacity, it is necessary to control the micro/meso porous structure and the hydrophilic/hydrophobic surface affinity. It is expected that the synthetic imogolite might become a multipurpose adsorbent.