This paper presents a thermodynamic isotherm derived from solution thermodynamics principles to describe gas adsorption on microporous materials. This isotherm relies on a potential relationship between the integral free energy of adsorption relative to saturation, Psi/RT, expressed by the Kiselev equation, and the variable Z = 1/ - Ln(Pi), being Pi the relative pressure. A mathematical analysis reveals that the adsorption energy heterogeneity in the micropores is collected in a characteristic parameter of the isotherm. m, that can be related to the alpha parameter of the Dubinin-Astakhov isotherm in a simple way (m = alpha + 1). The isotherm also predicts a plateau in Psi/RT at extremely low pressures (Pi < 10(-7)). Neimark's thermodynamic equation accounting for gas adsorption on mesoporous solids is found to be a particular case of the isotherm presented in this study. The Langmuir isotherm only shows consistency with the thermodynamic isotherm for a reduced combination of values of the relevant parameters, not usually found in common adsorbents. The suitability of the thermodynamic isotherm is experimentally assessed by testing a collection of microporous materials, including activated carbons, carbon nanotubes, and zeolites. (C) 2008 Elsevier Inc. All rights reserved.