The hydrogen adsorption isotherms of six metal-organic frameworks (MOFs) and three microporous carbons, measured at 77 K (up to 1 bar) and 298 K (up to 100 bar), have been systematically examined for correlations with their pore characteristics. From the obtained correlations, H 2 adsorption was found to occur preferentially in ultrafine pores at both 77 K (1 bar) and 298 K (100 bar), irrespective of the adsorbent. This represents the first experimental evidence that ultrafine pores in MOFs improve the efficiency of H 2 adsorption at 298 K and at high pressures, indicating that that the low H 2 storage capacities of reported ultrahigh microporous MOFs at 298 K result from the prominence of micropores with diameters 1-2 nm, which are inadequate at 298 K and high pressures. Furthermore, these correlations suggest strong links between the H 2 storage capacities at 77 and 298 K, which offer an easy method for predicting H 2 adsorption capacities under unapproachable conditions. This study provides guidance in the development of new MOFs or other adsorbents with an optimized H 2 storage capacity at near-ambient temperatures and a swift screening method of newly synthesized porous adsorbents.