The three metabolites hydroxyibuprofen (OH-Ibu), carboxyibuprofen (CA-Ibu), and carboxyhydratropic acid (CA-HA), also known from human metabolism of ibuprofen, could be identified in biodegradation experiments. Identification was based on El mass spectra and comparison with literature data. Detection was performed by selective MS-MS measurements by GC-ion-trap MS and online methylation. Ibuprofen (Ibu), OH-Ibu, and CA-Ibu could be detected with a signal-to-noise ratio of 10:1 at a concentration of 2 nmol L-1, CA-HA at 0.5 nmol L-1. Degradation experiments in both biofilm reactors (BFR) and batch experiments with activated sludge (BAS) reveal OH-Ibu as the major metabolite under oxic conditions, and CA-HA under anoxic conditions. CA-Ibu was found under oxic and anoxic conditions almost only in the BAS. The metabolites together do not account for more than 10% of the initial concentration of Ibu.