The removal of imidacloprid (IMI) from water by ozonation (O-3) and photo-ozonation (O-3/UV) was comparatively studied, paying particular attention to the kinetics, matrix effect, and mechanistic aspects of the processes. The IMI removal by O-3 was considerably enhanced at alkaline pHs, leading to almost complete removal under 20 min with a pseudo-first-order rate constant of 0.2374 min(-1) at pH 8.25. Three different matrices, Milli-Q water, full-scale vacuum rotating membrane bioreactor plant effluent (VRMBR WW), and laboratory-scale instantaneous fed-batch reactor bioreactor effluent (Bio WW) spiked with IMI, were tested. The ozonation, coupled with UV, improved IMI removal remarkably regardless of the wastewater matrix, and there occurred a six times decrease in ozonation time requirement for 99% IMI elimination at pH 7.25. The IMI degradation mechanism proved that IMI is an ozone-resistant pollutant and is mainly degraded by OH center dot via an indirect mechanism. The second-order rate constants for IMI degradation with OH center dot were calculated as 2.23 x 10(11) and 9.08 x 10(11) M-1 s(-1) for the O-3 alone and O-3/UV processes, respectively. The IMI degradation pathway analysis showed that IMI lost NO2, HNO2, and then Cl- from its structure, and the O-3/UV process yielded fewer by-products than O-3.