The influence of the framework density, the chemical composition, and derived from those the influence of the effective electronegativities of silica and aluminophosphate molecular sieves on the adsorption properties have been investigated experimentally and theoretically. By using the electronegativity equalization method (EEM), the average effective electronegativities have been determined. A correlation of the initial heats of adsorption (as measured by calorimetry) of nonpolar molecules such as ethane, n-hexane, and benzene is found with the framework density as well as with the effective electronegativity of the uncharged frameworks. An increasing density of the microporous solids and a corresponding decrease of the effective electronegativity results in an increase of the initial heats of adsorption. Because of a compensation effect of the entropy, the free enthalpy of adsorption increases less with rising framework densities than the enthalpy of adsorption does.