The transformation products 2-(isopropylcarbamoyl)phenylsulfamic acid and 2-(1-hydroxypropane-2-yl)-1,2-dihydroindazol-3-one could be determined during the photolysis of the herbicide bentazone. Degradation experiments were carried out with different types of water in a natural sunlight simulating system. Besides the anticipated hydroxylated bentazone, the second transformation product was identified by means of exact mass measurement using ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC/QqToF MS). Both phototransformation products occurred in all water types tested. The required irradiation time was matrix dependent. 2-(Isopropylcarbamoyl)phenylsulfamic acid was detected in a drainage channel in the Ebro river delta (Catalonia, Spain).