Organic substances in the influent and in the effluent of a fixed bed biology located at a lubricating oil refinery were characterized by gas chromatography/mass spectrometry (GC/MS). Totals of hydrocarbons were 0.8 mg/l (influent) and 0.1 mg/l (effluent) indicating a biological removal of nearly 90% of the hydrocarbons. High temperature gas chromatography/high resolution mass spectrometry of 1,1,2-trichloro-trifluoro-ethane extracts revealed that monocycloalkanes were the major group of hydrocarbons in the influent (590 µg/l) as well as in the effluent (65 µg/l) of the biological stage. A small amount of the hydrocarbons in the influent were aromatic compounds (mainly alkyl benzenes and benzocycloalkanes) which were degraded in the bioreactor to a lower extent than the aliphatic compounds. In dichloromethane extracts of the 60-fold concentrated effluent of the biological stage only few organics were identified gas chromatographically. Besides 1-methyl-2-propyl-cyclohexane a couple of ethers and nitrogen-containing heterocycles were the main components detected in the dichloromethane extracts. The data presented here confirm that aerobic fixed bed biology is an efficient final treatment step for refinery wastewaters.