Ibuprofen (IBU) and diclofenac (DCF) are non-steroidal anti-inflammatory pharmaceuticals. They are frequently detected in wastewater and in the environment. Due to their harmful effect on aquatic organisms, new methods should be developed to enhance their removal from wastewater. One efficient wastewater treatment process is ozonation. In this study the fate of IBU and DCF during ozonation was investigated. Transformation products were identified with liquid chromatography (LC) coupled to an ion trap mass spectrometer (MS) and a quadrupole time-of-flight MS. Also, gas chromatography – mass spectrometry was used in order to investigate the presence of more volatile transformation products than those detected with LC-MS. Ten different products could be identified for IBU, including four isomers of hydroxylated IBU. Six were conclusively identified with authentic standards. For DCF, 14 different products could be identified of which four were isomers of hydroxylated DCF. Three products were identified with authentic standards. IBU, DCF and the major ozonation products were quantified with LC coupled to a triple quadrupole MS. Both IBU and DCF can be efficiently be transformed via ozonation. Only 20% of IBU is left after 30 min, while 10 min is required to transform DCF to the same extent. The major product formed during the ozonation of IBU was IBU with an additional ketone group, while the major product formed from DCF was 4ʹ-OH-DCF.