AbstractWe have investigated the effect of added Si02 on the photochemistry of anthracene, 1, in cyclohexane. Adsorption of 1 onto the Si02 surface from cyclohexane follows a Freundlich adsorption isotherm indicating interactions between 1 and the surface over a distribution of adsorption sites. In the absence of oxygen, addition of Si02 to a cyclohexane solution of 1 leads to a significant increase in the rate of photolysis without a large concomitant change in product identity. The 9,10‐photodimer of 1 is the major product under oxygen‐deficient conditions. In the presence of oxygen, added Si02 has dramatic effects on both the photoproduct distribution and the kinetics of photodecomposition. The complicated mixture of oxygenated products formed from photolysis in oxygen‐containing slurries is proposed to arise from thermal decomposition of a single primary photoproduct, anthracene‐9,10‐endo‐peroxide. The polarity/polarizability of the Si02/cyclohexane interface is considered to be a major factor influencing the rate of photolysis in the oxygenated Si02/cyclohexane slurries.