Alternative exposure of Pd thin films and Pd/TiO2 catalysts to dissolved hydrogen and oxygen leads to significant changes in the reflectivity of infrared radiation as observed in attenuated total reflection spectroscopy. The reflectivity decreases and the absorbance increases upon changing from oxygen- to hydrogen-saturated solvent. Reflectivity calculations based on the Drude model for the Pd thin film show that a slight change in the concentration of the free electrons of the metal could be at the origin of the observed effect. Alternatively, the reversible formation of a surface oxide layer can lead to a similar observation. The reflectivity changes can be used to follow the changes of the metal catalyst, similar to potential measurements, however without the need to work in conducting media. They can be correlated with the observation of adsorbed species and the formation of reaction products. The potential of the method for in situ studies of catalytic solid−liquid interfaces is demonstrated for the oxidation of 2-propanol and ethanol. Upon changing from reducing to oxidizing conditions, the observation of reaction products is slightly offset with respect to the observed reflectivity change in both cases, whereas the frequency of the CO vibration shifts at the same time as the reflectivity increases.