Adsorption equilibrium isotherms of benzene, toluene, and p-xylene on poly(isobutylene) (PIB) gel are experimentally measured at (293.15, 303.15, and 313.15) K on a quartz crystal microbalance to obtain very low-pressure adsorption data near the Henry's law region. This apparatus is very useful in measuring adsorption amounts on nonporous adsorbents, especially those with small surface areas such as PIB, which has been extensively used as a basic coating material. The experimental data are correlated with the Henry and Langmuir equations. The Henry's law constants determined in this work are discussed to assess their feasibility as thermodynamic values.