The construction of an apparatus for the rapid measurement of the adsorption equilibrium characteristics and evaluation of the isosteric heat of adsorption of activated carbons for gaseous adsorbates is described. The method is based on measuring the amount desorbed between two equilibrium adsorption states. The measurement system has been tested for methane adsorption over (5 to 75) degrees C and pressures from (2 to 24) bar on a Maxsorb-III specimen of activated carbon. The results are evaluated through regressions with various isotherm equations used in the literature for physisorption and are compared with results of other researchers for comparable adsorbent-adsorbate combinations. A method of accounting for the adsorbed phase volume is proposed.