Three OH stretching bands, with maxima at about 3745, 3650 and 3545 cm–1, are present in infrared spectra of decationated X- and Y-type zeolites. Pyridine reacted with the 3650 cm–1 species to form pyridinium ion. This band underwent a hydrogen-bonding shift when ethylene was adsorbed at room temperature, indicating a strong physical adsorption which was quantitatively and rapidly reversible on raising or lowering the pressure. No exchange occurred between C2H4 and the OD groups of an exhaustively deuterated zeolite. The hydrogen-bonding shift from 3650 to 3300 cm–1 was identical when C2D4 was substituted for C2H4. The 3745 and 3545 cm–1 bands were not noticeably altered by the adsorption, at least in the pressure region where the 3650 cm–1 band was affected. Adsorption isotherms, calculated from the spectra, could be superimposed on those determined by volumetric techniques. Both were close to Langmuir-type but showed evidence of a superimposed non-selective adsorption at higher pressures. Ethylene may also adsorb, via a charge-transfer interaction, on the residual Na+ ions of the zeolite. At about 500 torr, the total adsorption of C2H4 was about 30 % greater than the available zeolite hydroxyl groups. It was inferred that the strongest interaction was with the acidic hydroxyl groups and that, at low pressures, most of the adsorbed C2H4 was hydrogen-bonded to these. In the CH bending and stretching regions, only the Q-branch of the spectra of the gaseous molecules appeared, indicating that molecular rotation was hindered in the adsorbed state. Qualitatively similar results were obtained with C3H6. The hydrogen-bonding shift was greater by about 100 cm–1, however, and the adsorption was not entirely reversible. Moreover, the irreversible portion increased with time, and products stemming from polymerized olefin could be recovered; also, C3D6 exchanged with the OH groups of the 3650 and 3545 cm–1 bands, but not with those of the 3745 cm–1 band. Preliminary results for these substrates adsorbed on dehydroxylated, decationated zeolites are given.