Differential enthalpies of adsorption (Delta H-ads) of benzene, ethylbenzene, and 1,3- and 1,4-diethylbenzene onto zeolite H-Y at 323 K were measured as a function of sorbate coverage using a microcalorimeter of the Tian-Calvet type. The Delta H-ads values which correspond to interaction with acidic Bronsted sites varied between the various probe molecules in the range 66 to 125 kJ mol-l (neglecting the initial high enthalpies) and increased in the following order: benzene < ethylbenzene < 1,4-diethylbenzene approximate to 1,3-diethylbenzene. Benzene exhibited a constant Delta H-ads Over a wide range of sorbate loadings, while the substituted benzenes showed gradual increases in Delta H-ads With coverage, indicating mutual interaction between sorbate molecules. The total number of molecules which were adsorbed per unit mass of zeolite varied inversely with the size of the sorbate molecule, in accordance with volumetric limitations. High values of Delta H-ads were evolved on sorbing the first few molecules of any of the sorbates per unit cell, due to the interaction between the sorbate and small numbers of residual Na+ cations in the zeolite. The differential heats of adsorption of 1,4-diethylbenzene onto dehydroxylated H-Y, as well as partially sodium-exchanged H-Y, were measured for comparison.