Membrane inlet mass spectrometry (MIMS) has been developed for the on-line monitoring of compounds in a continuous stirred tank bioreactor (CSTB) used to simulate a wastewater treatment plant (WWTP). A mixture of four industrially relevant nitrogen-containing volatile and semi-volatile compounds was fed to a 3dm(3) CSTB with a hydraulic retention time (HRT) of 24h. In-membrane preconcentration/thermal desorption was used for the on-line monitoring of semivolatile compounds (n-methylpyrrolidinone and tetramethylethylenediamine), while volatile organic compounds (3-bromopyridine and 2-chloro-5-trifluoromethylaniline) were determined by continuous infusion through the membrane. Quantification of the four compounds was achieved by MS. The CSTB was run for 160 h and, after reaching steady state, n-methylpyrrolidinone showed removal to levels below the limit of detection, while there was partial biodegradation of 2-chloro-5-trifluoromethylaniline and 3-bromopyridine. There was no evidence of significant biodegradation for tetramethylethylenediamine. Intermediates in the metabolite pathways of 2-chloro-5-trifluoromethylaniline and 3-bromopyridine were identified by MIMS, gas chromatography/mass spectrometry (GC/MS) or liquid chromatography/mass spectrometry (LC/MS). COD was measured off line, and results agreed well with MIMS, although COD data did not provide information on the removal of the individual compounds. (C) 2003 Society of Chemical Industry.