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Wastewater treatment is an important source of methane (CH4) emissions. Inmost large-size aerobic treatment plants, the excess sludge is digested in anaerobic reactors (AD), with the concomitant CH4 emissions. The guidelines of the Intergovernmental Panel on Climate Change (IPCC) have been adopted worldwide for quantifying the national emission inventories, which include wastewater treatment plants (WWTP) as a key category. The IPCC recommends using default emission factors (Tier 1) for countries with limited available data (such as Mexico and most developing countries). However, these estimates have a high degree of uncertainty, owing to the lack of reliable information about the operation process and local environmental conditions. In order to reduce uncertainty in the estimation of CH4 emission from WWTP in Mexico, a country-specific emission factor was determined for AD associated with activated sludge process. This was accomplished with on-site data obtained from the AD of six activated sludge WWTP. In addition, the measured CH4 emissions were compared to those resulting from the application of the IPCC Tier 1 method, using the recommended default methane correction factor (MCF: 0.8) as well as alternate values (0.32 and 0.26) recently proposed by the authors. Results show that the IPCC Tier 1 method, using the recommended MCF, highly overestimate CH4 emissions compared with the values obtained on-site. In contrast, the alternate MCF achieved better estimations than the IPCC-recommended MCF, much closer to the observed emission values. The CH4 emission factor proposed as country (Mexico) specific value is 0.49 kg CH4/kg BODrem, which would allow the application of IPCC Tier 2 method. By doing so, the uncertainty associated with CH4 emission from aerobic treatment plants with AD would be reduced. This, in turn, would provide important information for implementing appropriate CH4 mitigation strategies for the water sector. (C) 2018 Elsevier B.V. All rights reserved.