The contamination by domestic sewage of the Iguaçu River and Sarapuí channel, located in the north-western sector of Guanabara Bay, was investigated by the determination of sterols and elemental composition (C and N) of organic matter in suspended matter and superficial sediments. Physico-chemical and chemical characterization of water and sediments were considered as well. Conventional analytical methods, like gas chromatography/mass spectrometry (sterols) and dry chemical oxidation (C and N), were employed after optimization for specific field and laboratory conditions. In March and September/2004 water/suspended particles (7 sample stations) and sediment (10 stations) were collected in a transect from 6 km up the river until 4 km off the river, in the bay. An additional sample of bulk sewage was also collected at a sewage treatment plant close to the studied area. The faecal sterols predominate over phyto sterols in suspended matter and sediments. Among the 14 sterols quantified, higher concentrations (up to 21 mg gC-1) were measured for coprostanone (5b-cholestan- 3b-one), epicholestanol (5a-cholestan-3a-ol) and coprostanol (5b-cholestan-3b- ol). In general, there is a river-bay decreasing gradient in sterol concentration, associated with the location of diffuse sources of sewage. Seasonal variation in climatic factor also influenced in the sterols distribution. The presence of epicholestanol (reported here for the first time in Guanabara Bay) in relatively high concentrations in suspended matter and sediments (as well as in raw sewage), the differences in sterol composition between raw sewage and suspended matter/sediments, and the values obtained for selected sterols source-diagnostic ratios, suggested the occurrence of significant microbial alteration of organic matter, with implications on the geochemistry of sterols. The elevated degradation