Recent research works show that wastewater treatment plants may contribute significantly to the total emission of volatile org. compds. (VOCs) in urban areas. In order to investigate this contribution, a measurement campaign was carried out during Sept., 1999, in the Lausanne, Switzerland, area. The concns. of monocyclic arom. hydrocarbons (MAHs) over a wastewater treatment plant at Lausanne were monitored by differential optical absorption spectroscopy (DOAS) and gas chromatog. with a flame ionization detector (GC-FID). An FID was used for monitoring the overall concn. of nonmethane hydrocarbons (NMHCs). The local meteorol. conditions were also measured. The concns. of benzene, toluene, ethylbenzene, xylenes, 1,2,4-trimethylbenzene, and phenol over the wastewater treatment plant as measured by DOAS and GC-FID are compared. In addn., based upon the results of a recent field expt., the impact of the oxygen interference on DOAS measurements of arom. compds. is addressed. The MAH concn. time series and its relation to that of NMHCs, as well as to the meteorol. conditions, are presented. The VOC release and prodn. phenomena involved in the wastewater and sludge treatment processes are analyzed using complementary measurements of arom. compds. in wastewater and gas processing streams.