A method for the determination of the heat of adsorption from solution of a substance, , from measurements of the equilibrium concentration of the substance remaining in solution as a function of the temperature is presented and compared with the direct measurement of the heat of adsorption using a state-of-the art heat conduction microcalorimeter for the system phenol in aqueous solution and an activated carbon surface. Equilibrium considerations are employed similar to those used to calculate the so-called isosteric heat of adsorption from the slope of the equilibrium pressure vs. the temperature at constant adsorption but in a way simpler than this method which requires recording the pressure as a function of the coverage, ξ, at several temperatures while the present method gives directly the heat of adsorption from the single slope of the equilibrium concentration vs. the temperature at constant pressure. The transfer of moles of solute from the solution to the carbon surface obeys Nernst’s distribution law with an equilibrium constant which depends only on the temperature and pressure. Hence, the quantities obtained from the equilibrium condition are standard functions. The values obtained for the heat of adsorption of phenol by this method are typical of those found for mild physical adsorption. Heats of adsorption are important to characterize porous carbon surfaces and for the study of intermolecular interactions in condensed phases