Sunlight irradiation of dissolved organic matter (DOM) leads to the formation of photochemically produced reactive intermediates, which can induce the indirect phototransformation of a variety of organic contaminants. A previously observed enhancement effect in the indirect phototransformation of electron-rich phenols at submicromolar concentration was ascribed to so far unexplored “long-lived” photooxidants (LLPO). Such LLPO, having estimated lifetimes τ>100 μs, are hypothesized to occur concomitantly with the shorter-lived excited triplet states of DOM, ³DOM* (τ~2 μs). We studied the relevance and role of LLPO by steady-state irradiations, chemical model systems and kinetic modelling. By comparing the transformation rate constant at a high (5.0 μM) and a low (0.1 μM) initial concentration of target compounds, LLPO were observed to enhance the transformation of electron-rich compounds out of various chemical classes, namely phenols, anilines, phenylureas and sulfonamides. Chemical model systems consisting of a photosensitizer and an electron-poor phenol produced an analogous enhancement effect on transformation rates of electron-rich compounds, supporting the hypothesis that electron-poor phenolic moieties of the DOM are plausible LLPO prescursors.