The effect of 0 2 on the photolysis of aromatic model compounds adsorbed on a silica gel surface is reported. The rate constant for the decay of pyrene (Py) fluorescence is employed as a monitor of 0 2 pressure in the irradiated system. Steady-state photoirradiation of Pyhilica gel samples shows consumption of 0 2 , and the generation of a new absorption band and EPR signals, indicating reaction of 0 2 during photolysis. Timeresolved observations show that the pyrene radical cation (Py'+) is a reaction intermediate of the phototransformation. Low-temperature studies detect the presence of py'+, 0 2 ' -, and H ' . It is shown that the formation of 02.-can proceed via capture of trapped electrons on the silica gel surface and that the formation of H' is an 01-mediated process that is unique to the host, Le., the solid surface. Warming the irradiated samples from -140 "C leads to a decrease in the EPR signal and a disappearance of H ' . The reaction of MV" with 0 2 is observed by EPR measurements in steady state and by diffuse reflectance in a time-resolved mode. The EPR studies show that the product is 02*-, and the rate constant of reaction of 0 2 with M V on the silica gel surface is determined as 2.5 x lo5 M-I s-l. The study shows some similarities between the photochemistry of organic molecules in polar liquids compared to a silica gel surface. However, the surface promotes additional features of the photochemistry, in particular, pronounced two-photon photoionization and the 0 2 -mediated transformation of etr-to H' atoms. These effects either do not occur in solution or required very high light intensities.