Abstract The inclusion of a number of representative molecules (He, Ne, Ar, Kr, Xe; H2, O2, N2; n-C4H10,i-C4H10, C6H6, neo-C5H12) has been studied for Na- and H- forms of synthetic mor-denite. Filling of the intracrystalline volume in Na-mordenite is limited to some of the permanent gases, and at 25 °C is very incomplete for n-C4H10. In contrast, in H-mordenite the main channels are nearly filled by molecules as large as benzene and neo-pentane. However, sorption in the sets of small cavities or pockets which line the main channels in mordenite is limited to molecules smaller than n-butane. Heats of inclusion have been determined as functions of amounts sorbed for the inert and the permanent gases. Calculations have been made of heats of inclusion of the rare gases in the small cavities, and compared with observed initial heats in H-mordenite. As in the case of gas hydrates (Barrer & Ruzicka 1962) the London (1930) approximation to the dispersion energy, combined with the Lennard-Jones 12:6 potential, gives better results than the approximation of Kirkwood (1932) and Muller (1936). Estimates have also been made of equilibrium constants of rare gases in the small cavities, which give a reasonable correlation with the measured Henry’s law adsorption constants. Conclusions are drawn from certain of the results about the separability of intracrystalline and inter-particle sorption. There is also reason to believe that stacking faults are not the cause of the molecular sieve character shown by the synthetic Na-mordenite.