The development of high-performance materials for hydrogen storage by adsorption requires detailed understanding of the adsorbate-adsorbent interactions, e.g., the enthalpy of adsorption Delta H, which measures the interaction strength. The determination of Delta H for a weakly adsorbing gas such as hydrogen in a carbonaceous porous material is difficult experimentally, normally requiring measuring two cryogenic adsorption isotherms. Here we demonstrate a calculation of Delta H based on ca. room temperature adsorption isotherms at 273 K and 296 K using the Clausius-Clapeyron equation. This requires an estimation of the volume of the adsorbed film (similar to 40%, similar to 12% of the total pore volume at 77 K, 296 K, respectively) obtained from fits of the excess adsorption isotherms to an Ono-Kondo model with the auxiliary use of a fixed point corresponding to the saturation film density (estimated as 100 +/- 20 g/L) which appears to be remarkably sample and temperature independent, i.e., a property of the adsorbate. The calculated room temperature enthalpy of adsorption Delta H = 8.3 +/- 0.4 kJ/mol is in excellent agreement with the low-coverage cryogenic determination of Delta H. The methodology hereby proposed facilitates reliable calculations of the enthalpy of adsorption at room temperatures for weakly-adsorbing gases. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.