The potential theory of adsorption has been modified, taking into account the adsorbate compressibility. A new equation has been obtained for adsorption isotherms in general cases. A particular case of microporous adsorbents such as active carbons is considered in detail. The Dubinin-Radushkevich equation has been corrected. The estimations, with the example of benzene adsorption on a model typical microporous adsorbent, show that ignoring the adsorbate compressibility leads to mistakes in the calculation of adsorption isotherms via the conventional Dubinin-Radushkevich equation of about 10% at room temperature, and this difference increases with an increase in the temperature. This paper is a development of the previous paper (Commun. Acad. Sci. USSR (Chem.) 1, 13 (1992)), in which attention was paid mainly to calculating the micropore volume from the experimental value of the limit of adsorption.