Photochemical transformation of hydroxylated polyhalodiphenyl ethers (HO-PXDEs) has attracted much attention for their ubiquitous presence and the photochemical formation of highly toxic dioxins. Dissolved organic matter (DOM) plays an important role in the environmental photochemical transformation of organic pollutants. However, the effects of DOM on the photolysis kinetics and dioxin formation of HO-PXDEs are still not fully understood. Herein, the effects of Suwannee River natural organic matter (SRNOM) on the phototransformation of 2'-HO-2,4,4'-trichlorodiphenyl ether (triclosan) and 2'-HO-2,4,4'-tribromodiphenyl ether (2'-HO-BDE-28) were investigated in artificial estuarine water (AEW). The results showed that although SRNOM induced indirect photolysis of triclosan and 2'-HO-BDE-28, it decreased the observed photolytic rate constants due to light screening, static quenching and dynamic quenching effects. The above effects were quantified firstly. Direct photolysis is more important than indirect photolysis in the transformation of the target compounds and the production of dioxins. SRNOM increased the dioxin yields of the two HO-PXDEs. It was also found that SRNOM decreased the formation rate constant (kp) of 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) from triclosan and showed no obvious influence on the kp of 2,8-dibromodibenzo-p-dioxin (2,8-DBDD) from 2'-HO-BDE-28. SRNOM showed no obvious influence on the degradation of 2,8-DCDD, while it increased the degradation rate constant of 2,8-DBDD. The promoting effect on the degradation of 2,8-DBDD was attributed to the formation of chloride radicals with the concurrence of SRNOM and Cl(-) in AEW. This study revealed the roles of SRNOM in the photochemical transformation of HO-PXDEs and the photochemical formation and degradation of dioxins, which is important for elucidating the transformation fate of HO-PXDEs in aquatic environments.