By application of the statistical theory of the radial distribution function and the three-dimensional lattice gas theory, an Ar encapsulation isotherm curve of dehydrated sodalite up to 1000 atm at 534 K and CH4, C2H4, Ar, and Kr encapsulation isotherm curves of dehydrated 40% potassium-modified zeolite A up to 4300 atm at 523 and 623 K were calculated. The gases used were approximated as Lennard-Jones gases. Comparison of these calculated data with known experimental data showed a good agreement, particularly in the systems involving gas molecules which have a spherical shape such as CH4, Ar, and Kr. However, the results from the system involving the nonspherical molecule C2H4 were not satisfactory. It is believed that this is mainly due to the anisotropy of the C2H4 molecule.