Nitrobenzene (NB) pollution of the Songhua River caused by an explosion at a petrochemical plant in Jilin City, Jilin Province, China, attracted public conc over the fate of NB in the river. As one of the efforts to predict the fate of residual NB in the river, the kinetics and mechanism of phototransformation of NB in natural water from four sections of the Songhua River were investigated. It was found that photodegradation of NB in water proceeded via pseudo-first-order reaction kinetics under simulated solar irradiation conditions, and the rate constant (k) in the ultrapure water system was 0.0168 h-1, which was approximately half of those in the natural river water systems (0.323-0.402 h-1). The relatively short half-life of NB in natural river water systems (17.2-21.5 h) indicates that photodegradation might have played an important role in the loss of NB flux in the river. Nitrate concentration and alkalinity were the two main factors affecting the photochemical fate of NB, suggesting that decomposition of NB mediated by ·OH was predominant in water solution with high nitrate concentrations. Three isomeric nitrophenols, o-, m-, and p-nitrophenols, as well as phenol were detected as the main organic products in natural water under natural and simulated solar irradiations.