The partial TiO2-mediated photocatalytic degradation of anisole has been studied using isotopically labeled substrates and comparisons to Fenton and hv/H2O2 chemistry. An H/D isotope selectivity of 2.7 is observed for hydrogen abstraction from the methyl group in all cases. An O-18 tracer study shows that anisole is simultaneously converted to phenol by both ipso arrack and degradation of me methyl, The isotope selectivity for hydroxylation of anisole is reported to be about 1.3 under all conditions. This is attributed to an O-2-dependent competition along the: reaction pathway that can be avoided by use of benzoquinone,instead of O-2 as an electron acceptor with TiO2. Under those conditions, the H/D isotope selectivities are between 0.9 and. 1.0, as would be expected for inverse secondary kinetic isotope effects.