Adsorption behavior of the hydrocarbons n-hexane, n-octane, cyclohexane, neopentane, and benzene on microporous activated carbon fibers (ACFs) was studied by use Of molecular adsorption and adsorption calorimetry. The calorimetric aspects were discussed in connection with pore characteristics determined by nitrogen adsorption. The differential heats of adsorption of hydrocarbon, as well as the integral heats on ACFs of narrower pore widths, tended to become higher than those of wider pore widths, which is ascribed to micropore filling. These experimental results were also compared with the theoretical profile of the intermolecular interaction between adsorbates and adsorbents.