Using IR spectroscopic data of Cr(CO)6 absorbed on different surface centers on gamma-Al2O3, we were able to calculate isotherms for adsorption on these centers. Linear relations were observed between spectroscopic data (position, extinction coefficient, and halfwidth values) of corresponding bands of Cr(CO)6, adsorbed on different Lewis acid centers and the free enthalpy of adsorption. Thus it is possible to determine Delta Gad^a values for different centers from one adsorption measurement and thus to characterize the Lewis is acid strength of the centers.