The work presented herein discusses the potential of liquid chromatography-quadrupole-time-of-flight mass spectrometry (QqToF-MS) for the chemical structure elucidation of pharmaceutical degradation products (DPs). The model compound, the nonsteroidal anti-inflammatory drug diclofenac was subjected to microbiological transformation in a laboratory scale pilot wastewater treatment plant (WWTP) and its transformation products were detected in the outlet samples. Their chemical structures were elucidated using the principal features of the instrument, i.e. high resolution, accurate mass and MS/MS capability. A hydroxy-diclofenac, a benzoquinone imine derivative and a nitro analogue of diclofenac were successfully identified. The final structural elucidation of the fourth transformation product was not successful. Overall, the study emphasises the capabilities and potential of quadrupole-time-of-flight mass spectrometer showing it to be a powerful tool in both structure elucidation and confirming the identity of environmental contaminants.