Adsorption isotherms are reported for HZSM-5 and silica gel using a series of gas adsorptives at several temperatures above their critical temperature. The data are analyzed with a new multiple equilibria adsorption model producing equilibrium constants (K-i), capacities (n(i)), and enthalpies (-Delta H-i) for each of the processes. Unlike the amorphous adsorbents studied earlier, which contain a distribution of pore sizes, zeolitic materials have uniform pore dimensions. This uniformity provides a test of our interpretation of the number of processes required in the isotherm fits of the multiple equilibrium analysis, MEA. As expected from the two pores in the structure of HZSM-5, most adsorbates require two processes (K-1,K-ads and K-2,K-ads) to fit the adsorption isotherms. HZSM-5 is compared to amorphous carbonaceous adsorbents, revealing fundamental differences in their behavior. Small cylindrical channels in HZSM-5 lead to an unfavorable entropic contribution from restrictions imposed by adsorptive packing and interactions with the channel walls. The pores of the carbons studied (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K. J. Phys. Chem. B 1997, 101, 7548-7555) are slit-shaped, leading to less restriction and larger equilibrium constants. For several adsorbates, the greater enthalpic interactions in the small HZSM-5 pores are accompanied by lower equilibrium constants than in the larger pores because of unfavorable entropic contributions. Finally, the larger total micropore volumes of the carbons studied for these adsorptives (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K. J. Phys. Chem. B 1997, 101, 7548-7555) result in increased capacity compared to HZSM-5. The process capacities from MEA (mol g(-1)) are converted to pore volumes using the molar volume. Surface areas are calculated from molecular areas of the adsorbates. Pore volumes and surface areas calculated from the process capacities are compared to those from conventional N-2 porosimetry and are shown to provide a more detailed and more accurate assessment of areas and volumes. These results show that MEA has the potential of becoming a standard characterization method for microporous solids that will lead to an increased understanding of their behavior in gas adsorption and catalysis.