Nano-α-Fe 2 O 3 can accelerate the degradation of diethyl phthalate ester (DEP) in the presence of citric acid under UV (300–400 nm) irradiation, but the underlying mechanism remains unclear. In this study, based on ultraviolet photoelectron spectra (UPS) and density functional theory (DFT) analyses, it was found that thermodynamically, a lower bandgap energy ( E bandgap = E CB -E VB < 5.8 eV) of the complex of CA-α-Fe 2 O 3 predicted its higher catalytic ability than citric acid ( E bandgap = E LUMO -E HOMO = 7.0 eV). X-ray photoelectron spectroscopy (XPS) study indicated that citric acid was chemically adsorbed on the surface of α-Fe 2 O 3 through its carboxyl group by filling the surface oxygen vacancies of α-Fe 2 O 3 (disappearance of Fe 2p3/2 peak at 711.5 eV). UV light induced a succession of reactions which resulted in heterogeneous photo-Fenton-like reactions that produced ·OH for DEP degradation. The dissolved oxygen was proposed to be the initial oxidant forming the reactive oxygen species. The solution pH, citric acid and α-Fe 2 O 3 concentration substantially influenced DEP degradation. The method in this study can be further applied to study other organic acids and metal oxides catalysts. [ABSTRACT FROM AUTHOR]