In this study, the results of odour concentration measurements on different wastewater treatment plants are presented and used in order to estimate the odour emission factors relevant to single odour sources. An odour emission factor is a representative value that relates the quantity of odour released to the atmosphere to a specific activity index, which in this case was the plant treatment capacity, resulting in an odour emission factor expressed in odour units per cubic metre of treated sewage. The results show that the major odour source of a wastewater treatment plant is represented by the primary sedimentation (with an OEF equal to 1.9 x 10(5)ou(E) m(-3)). In general, the highest OEFs are observed in correspondence of the first steps of the wastewater depuration cycle (OEF between 1.1 x 10(4)ou(E) m(-3) and 1.9 x 10(5)ou(E) m(-3)) and tend to decrease along the depuration process (OEF between 7.4 x 10(3)ou(E) m(-3) and 4.3 x 10(4)ou(E) m(-3)). In general, the OEFs calculated according to this approach represent a model for a rough prediction of odour emissions independently from the specific characteristics of the different plants.