Complete mineralization of 4-chlorophenol in water can be achieved by photocatalytic degradation of oxygenated solutions containing suspended TiO2. The chemical pathways of this degradation are complex, and in this paper, that which begins with hydroquinone is examined. Hydroxylation to form 1,2,4-benzenetriol is the first step, though a very small amount of cleavage of the C1−C2 bond is observed. The first major group of acyclic compounds derives from oxidative cleavage of either the C1−C2 or C3−C4 bond of 1,2,4-benzenetriol. It is argued that this results from single electron oxidation and capture by superoxide. Many smaller compounds have also been identified, and routes to various ones of them are proposed. Nearly all of the compounds are verified by use of authentic samples.