Abstract Thermodynamic analysis of the temperature dependence of adsorption of individual substances has been carried out. The concept of isoergs, i.e., lines of constant value for the free-adsorption enthalpy, has been introduced. It is shown that in coordinates (In p - a), the abscissa of the inflection point of the adsorption isotherm a infl is temperature invariant, and the ordinate of the inflection point in p infl depends linearly on the reciprocal temperature; a single isotherm is sufficient for finding the parameters of this dependence. An adsorption isotherm equation has been proposed, which is a modification of the Hill-de Boer equation. For this expression, a thermal equation has been obtained, all of whose parameters can be determined from a single adsorption isotherm. This equation describes adsorption equilibria well over a very wide range of pressures, temperatures, and fillings.