Abstract: The transformation of two X-rays contrast media iopromide (IOPr; non-ionic) and diatrizoate (DTZ; ionic) in reverse osmosis (RO) brines during UVA-TiO2 oxidation was investigated. The scavenging effect of chlorides, alkalinity and fulvic acid (HA) was comparatively evaluated against ultrapure water. The transformation of both compounds followed a pseudo-first reaction order, but rate of IOPr oxidation was higher than DTZ regardless of the water matrix. In ultrapure water, 98% IOPr transformation was achieved compared to only 42% for DTZ after a 6 h experiment. In contrast, transformation was significantly reduced in RO brines with only 18 and 9% transformation for IOPr and DTZ, respectively, whereas chlorides displayed very low to none scavenging effect, fulvic acid and alkalinity decreased rate constants by about 50% due to competitive radicals scavenging. The scavenging effect of HA was more evident for DTZ than IOPr. The ratio of deiodination/transformation was higher in DTZ, indicating that the initial transformation involves deiodination, e.g. takes place predominantly in the triiodinated skeleton. In contrast, the ratio of deiodination/transformation was lower in IOPr, indicating that the initial transformation proceeds without deiodination, e.g. takes place predominantly in the side chains. Finally, in both cases, kinetic experiments showed that oxidation leads to iodide evolution without iodate formation.