This paper presents new results on the characterisation of leak rates in polyethylene gas distribution systems. The work was motivated by the need to update the emission factors used to estimate methane emissions in Spanish networks. There are included a survey of the emission factors accepted in other countries as well as a detailed characterisation of the leak rates in polyethylene medium pressure mains. The pressure variation method was selected for the field tests, which were fulfilled in 51 Spanish and 13 Mexican sites. The results obtained confirm that the average emission factor for polyethylene gas distribution networks working at medium pressure is 0.46 ± 0.15 Nm³ year⁻¹ m⁻¹. Based on the results of this study, a new calculation methodology has been developed. On the other hand, the experimental data reported here are thought to be representative of polyethylene medium pressure distribution lines in other countries, and might be useful to check existing leak calculation methods.