Environmental context. Nonylphenol polyethoxylates (NPEOs) are widely used non-ionic surfactants, and they cause environmental concern because some metabolites of NPEOs possess endocrine-disrupting activities. Photodegradation is an important pathway for NPEOs degradation, and different degradation products may lead to different environmental risks. The present paper looks at the kinetics and pathways of NPEO photodegradation in aqueous solutions, focussing on the effects of humic acid, H2O2, and FeIII. We found that the presence of different chemicals led to different degradation pathways, and a new mechanism is proposed. Abstract. To further elucidate the mechanism of photoinduced degradation of nonylphenol polyethoxylates (NPEOs) in aqueous environments, two different light systems, UVA and simulated sunlight, were used, and the effects of humic acid, H2O2, and FeIII were investigated. The 96-h degradation efficiencies of NPEOs in pure water solution were found to be 36.6 and 22.6% under UVA and SSL irradiation respectively. The presence of humic acid and FeIII in solution increased the photodegradation efficiency of NPEOs to different extents. The proportion of short-chain NPEOs in the NPEOn mixture was found to increase significantly in the solution containing FeIII, whereas this phenomenon was not observed in pure water and solutions containing H2O2 or humic acid. The result of NPEO3 photodegradation experiments indicated that FeIII in solution led to an ethoxylate-reduction pathway. Dicarboxylated formate ethoxylates were proposed as the intermediate products of NPEO photodegradation through an oxidative pathway based on the analytical results of liquid chromatography–electrospray ionisation–mass spectrometry and tandem mass spectrometry. Different mechanisms of NPEO photodegradation were elucidated.