EPR spectroscopy has been used to characterize the interactions between aldehydes and the surface of rutile TiO2 active toward heterogeneous charge transfer and ultimately the formation of surface peroxyacyl radicals. Paramagnetic centers formed by vacuum reduction and UV irradiation of TiO2 have been detected and characterized at the dehydroxylated particle surface. In low-temperature (100 K) gas-solid studies, reduced TiO2 surfaces have been used as model systems for interfacial charge-transfer processes. Radical products resulting from aldehyde reduction are not detected at 100 K on the reduced surface even under UV irradiation. However, coadsorption of aldehydes with O-2 and subsequent UV irradiation on reduced TiO2 surfaces does result in the formation of stable surface radicals at 100 K. When the reactions conditions are varied, such as aldehyde type and UV wavelength, these radicals were identified as peroxyacyl species (RCO3.) stabilized at the semiconductor surface. The RCO3. radicals were only formed in coadsorption experiments and decayed irreversibly at T > 250 K. The results clearly demonstrate that the TiO2 surface can participate in both the initiation and propagation stages of aldehyde photooxidation. Similar radical species were not detected in UV-irradiated TiO2-aldehyde suspensions at 100 K.