A field application of the radiocarbon (C-14) method was developed to determine the ratio of biogenic vs. fossil CO2 emissions from waste-to-energy plants (WTE). This methodology can be used to assign the Kyoto relevant share of fossil CO2 emissions, which is highly relevant for emission budgets and emission trading. Furthermore, heat and electricity produced by waste incinerators might be labelled depending on the fossil or biogenic nature of the primary energy source. The method development includes representative on-site CO2 absorption and subsequent release in the laboratory. Furthermore, a reference value for the 14C content of pure biogenic waste (f(M.bio)) was determined as 1.130 +/- 0.038. Gas samples for (CO2)-C-14 analysis were taken at three WTEs during one month each. Results were compared to an alternative approach based on mass and energy balances. Both methods were in excellent agreement and indicated a fraction of biogenic CO2 slightly above 50%. (c) 2007 Elsevier Ltd. All rights reserved.