Humic substances are known to affect the fate of organic chemicals in the environment. While their capacity to produce reactive species upon irradiation has been intensively studied, their inhibiting properties have been much less investigated. In the present work, we studied the influence of various humic substances (humic acids, fulvic acids, natural organic matter) on the riboflavin photosensitized transformation of 2,4,6-trimethylphenol which takes place via oxidation of the phenol by the triplet excited state of riboflavin. Between 2 and 20 mg L−1 humic acids show an inhibiting effect on this reaction while below 2 mg L−1, a small accelerating effect is generally observed. At 25 μM 2,6-dimethyl-1,4-benzoquinone also inhibits the photoreaction significantly. It is proposed that the quinone traps the superoxide anion produced in the course of the reaction yielding semiquinone radicals. The reduction potential of the quinone is low enough for making possible a subsequent reduction of 2,4,6-trimethylphenoxyl radical by semiquinone and a regeneration of 2,4,6-trimethylphenol. In the case of humic substances, the trapping of superoxide anion might be achieved by humic quinone moieties. In accordance, soil fulvic acids and aquatic natural organic matter which show a much lower electron accepting capacity than soil extracted humic acids do not show any inhibiting effect.