Characterization of adsorbent heterogeneities is a key issue in adsorption on microporous solids because microporosity has a substantial effect on sorption properties. Measurements of adsorption isotherms at low concentrations can be utilized to evaluate the adsorption-energy and micropore-size distributions. In addition to the specific surface area and the mesopore- and macropore-size distributions, the adsorption-energy and micropore-size distributions are recommended for characterizing the energetic and structural heterogeneities of microporous adsorbents. The methods for evaluating these distribution functions are discussed for different models of adsorption on microporous carbons.