To elucidate the roles of humic acids (HAs) and iron on the environmental fate and transport of organic pollutants in natural water, the interactions of HAs with Fe(III) were characterized by Fourier transform infrared spectroscopy (FTIR) spectra, Ultraviolet-visible (UV-vis) spectra and fluorescence spectra, indicating the formation of HAs-Fe(III) complex. Electron paramagnetic resonance (EPR) spectra show *OH radicals are generated and can participate in the photoreaction in solutions containing HAs, Fe(III) and HAs-Fe(III) complex. Under Xe lamp irradiation (lamda >290 nm), the photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D), as a kind of herbicide, followed the pseudo-first-order reaction kinetics. The pseudo-first-order rate constant of 2,4-D photodegradation with the presence of only 2,4-D (2 mg x L(-1)) was 0.007 h(-1). In the presence of HAs (5 mg x L(-1)), Fe(III) (0.2 mmol x L(-1)) and HAs-Fe(III) complex, the rate constants of 2,4-D degradation were 0.004, 0.034 and 0.046 h(-1), respectively. It was interesting to note that in the existence of HAs, 2,4-D photodegradation was inhibited. While in the presence of Fe(III), 2,4-D photodegradation was enhanced. Furthermore, in the coexistence of HAs and Fe(III), HAs-Fe(III) complex showed better increased effect on the photodegradation of 2,4-D than Fe(III) alone.