The time-dependent swelling and deswelling of Illinois No. 6 coal by pyridine were measured by combined NMR and NMRI techniques. The results indicate that pyridine transport in this coal proceeds via Case II diffusion for swelling and via Fickian diffusion for deswelling. Once the relative pyridine concentrations reach values between 0.65 and 0.45, the coal＇s physical state changes from a glassy to a rubbery state. Transitions are well recorded by an inflection observed in the trend of spin-lattice relaxation or spin-spin relaxation of pyridine, as the coal swells and deswells. NMRI data indicate that the coal swelling is anisotropic, with swelling in a plane perpendicular to the coal bedding plane being 13% greater than that parallel to the coal bedding plane; however, the process of deswelling is almost isotropic. The differences between the swelling and deswelling characteristics are discussed from the viewpoint of coal＇s intrinsic structural properties and stored geologic pressure effects.