Diffuse reflectance infrared spectroscopy experiments are described in which the concentrations of the reactant gases passing over a solid catalyst are modulated. This simple modification to the normal in situ experiment enables direct correlation of specific surface species with the production of gaseous products. Spectra of solely the active surface intermediates with no contribution from so called "spectator" species are generated. For CO oxidation over a Rh/alumina catalyst at 576 K, the active CO species is identified as linearly adsorbed CO an oxidized Rh sites (upsilon(CO) = 2100 cm(-1)) on a catalyst surface apparently otherwise dominated by CO adsorbed as unreactive geminal dicarbony.