The effect of O-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 O-2 pressure in the irradiated system. Steady-state photoirradiation of Py/silica gel samples shows consumption of O-2, and the generation of a new absorption band and EPR signals, indicating reaction of O-2 during photolysis. Time resolved observations show that the pyrene radical cation (Py(.+)) is a reaction intermediate of the phototransformation. Low-temperature studies detect the presence of Py(.+), O-2(.-), and H-.. It is shown that the formation of O-2(.-) can proceed via capture of trapped electrons on the silica gel surface and that the formation of H-. is an O-2-mediated process that is unique to the host, i.e., the solid surface. Warming the irradiated samples from -140 degrees C leads to a decrease in the EPR signal and a disappearance of H-.. The reaction of MV(.+) with O-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 O-2(.-), and the rate constant of reaction of O-2 with MV(.+) on the silica gel surface Is determined as 2.5 x 10(5) M(-1) s(-1). 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 O-2-mediated transformation of e(tr)(-) to H-. atoms. These effects either do not occur in solution or required very high Light intensities.