The degradation rates and kinetics of one commonly used iodinated contrast medium, iohexol, were investigated and compared during ultraviolet (UV) photolysis, UV/H 2 O 2 and UV/ S 2 O 8 2 − advanced oxidation processes (AOPs). Results indicate that the iohexol degradation rate increased in the order of UV/H 2 O 2 < UV irradiation < UV/ S 2 O 8 2 − and followed pseudo-first-order kinetics. Increasing persulfate concentration significantly increased iohexol degradation rate, whereas increasing H 2 O 2 concentration caused reverse effect. Radical scavenging test results show that UV photolysis, OH and radicals all contributed to iohexol degradation during UV/ S 2 O 8 2 − , but OH was the main contributor during UV/H 2 O 2 and was consumed by excess H 2 O 2. The kinetic models of iohexol degradation by both AOPs were developed, and the reaction rate constants with OH and were calculated as 5.73 (±0.02) × 108 and 3.91 (±0.01) × 1010 M−1 s−1, respectively. Iohexol degradation rate remained stable at pH 5–9 during UV irradiation and UV/H 2 O 2 , but gradually decreased at pH 5–7 and remained stable at pH 7–9 during UV/ S 2 O 8 2 −. The presence of anions displayed inhibitory effects on iohexol degradation during UV/ S 2 O 8 2 − in the order of Cl − > HCO 3 − ≫ S O 4 2 − . UV/ S 2 O 8 2 − AOP exhibited high degradation efficiency and stability on the basis of UV irradiation, which can be applied as a promising degradation method for iohexol. UV/ S 2 O 8 2 − AOP can effectively mineralize iohexol to CO 2 but promoted the generation of toxic iodoform (CHI 3), and the subsequent chlorination had the potential to reduce the content of disinfection by-products; therefore, further evaluation of possible environmental hazards is warranted. • The degradation efficiency of iohexol increased in the order of UV/H 2 O 2 < UV irradiation < UV/ S 2 O 8 2 − . • The iohexol degradation by UV/H 2 O 2 oxidation was mainly contributed by OH radical without UV photolysis. • The rate constants of iohexol reacting with OH and · S O 4 − were calculated as 5.73 (±0.02) × 108 and 3.91 × 1010 (±0.01) M−1 s−1, respectively. • pH has a negligible impact on iohexol degradation during UV irradiation and slight influence during UV/ and UV/H 2 O 2. • Higher concentrations of chloride and bicarbonate (≥2 mM) inhibited iohexol degradation. [ABSTRACT FROM AUTHOR]