The thermodynamics of nondissociative gas adsorption with two different states of adsorbate is considered. A general isotherm for the heterogeneous multi centered adsorbed state of gas at the gas/solid interface was developed. The new Azizian-Volkov (AV)-isotherm makes it possible to analyze adsorption and the interfacial structure in a general case. This AV-isotherm takes into account the interaction between adsorbed molecules and gives information about orientation of adsorbates. Traditional isotherms (Henry, Langmuir, and Frumkin) of the homogeneous adsorption can be derived from the AV-isotherm. Some examples of adsorption are considered, and the fitting results of AV-isotherm are compared with classical isotherms. (c) 2008 Elsevier B. V. All rights reserved.