A combined IR and MS technique was used to determine extinction coefficients of adsorbed CO on the 4% Rh/SiO2 catalyst. An isotopic exchange experiment has been made to obtain accurate integrated absorption intensities. The experimental procedure provides a fast and accurate, as well as a highly reproducible, method for the determination of extinction coefficients of adsorbed surface species. Integrated absorption intensities for Rh+((CO)-C-12) and Rh+((CO)-C-13)(2) were determined at 373 K and 0.1 MPa to be 1.35 and 8.02 cm/mu mol, respectively.