Adsorption isotherms of benzene, toluene, p-xylene, and mesitylene on high-silica MCM-41 (Si-MCM-41) at different temperatures (348-498 K) have been studied using gas chromatographic technique (GC peak maxima method). The Freundlich adsorption model fits the adsorption data for the aromatic hydrocarbons at all the temperatures. The isosteric heats of adsorption of the aromatic hydrocarbons at different loadings, obtained from the adsorption isotherms, are found to be in the following order: Q(a (benzene)) < Q(a (toluene)) < Q(a (p-xylene)) < Q(a (mesitylene)). The heat of adsorption of all the aromatic hydrocarbons is decreased with increasing the adsorbate loading; the decrease for the benzene adsorption at intermediate adsorbate loading is found to be more pronounced, indicating induced surface heterogeneity due to the presence of adsorbed benzene. Entropy analysis of the adsorption indicated that the adsorbed aromatic hydrocarbons (except benzene at very low adsorbate loading) are neither completely mobile nor localized; only the benzene adsorbed at very low adsorbate loading (less than or equal to 1.04 x 10(16) molecules m(-2)) is completely mobile or even supermobile. The mobility of the adsorbed toluene, p-xylene, and mesitylene is decreased (or increasingly restricted), but that of the adsorbed benzene is passed first through a minimum and then through a maximum, with increasing the adsorbate loading.