We undertook a comprehensive odour study for a municipal wastewater treatment plant as part of an environmental impact assessment. This included development of detailed source inventories and selecting specific sources for sampling and quantitative chemical analysis. Hydrogen sulfide and mercaptans at the parts-per-billion levels were determined with gas chromatography and flame photometric detection. The results were used to develop a mass emission inventory by source for dispersion modelling. The modelling results were compared with the odour complaint history and ambient air measurements.