It is demonstrated that cavity ringdown spectroscopy (CRDS) can be used to probe reaction intermediates desorbing from the surface during a heterogeneous catalytic reaction and provide information valuable in understanding the reaction kinetics. During water formation from H-2 and O-2, desorbed OH molecules outside of a polycrystalline platinum catalyst were quantified as a function of the relative hydrogen concentration, alpha(H2) using CRDS. The temperature of the catalyst was 1500 K, the total pressure was 26 Pa, and the flow was set to 100 sccm. At a distance of 6.5 mm from the Pt catalyst, the maximum OH concentration was found to be 1.5 +/- 0.2 x 10(12) cm(-3) at an alpha(H2) value of 10%, and the rotational temperature was determined to be 775 +/- 24 K. The desorbed OH molecules were also probed using laser-induced fluorescence (LIF), and the alpha(H2)-dependent OH abundance was compared with the CRDS results. The relative concentration of OH probed with LIF appeared to be lower at alpha(H2) = 30-50% compared to what was determined by CRDS. The observed discrepancy is suggested to be due to electronic quenching, as was indicated by a shorter fluorescence lifetime at alpha(H2) = 30% compared to at alpha(H2) = 10%.