Adsorption isotherms of methane, ethane, propane, n-butane, isobutane, carbon dioxide, and sulfur hexafluoride were measured gravimetrically on silicalite crystals at temperatures ranging from 3 to 81 degrees C and at pressures up to 2000 kPa. A virial equation was used to correlate the experimental data and to calculate the isosteric heats of adsorption and the limiting heats of adsorption at zero loading. The adsorption isotherms of isobutane exhibit inflection points at loadings of 4-6 molecules/unit cell in a certain temperature range. This unusual adsorption behavior is attributed to adsorption of isobutane in different locations of the channel system.