Furthermore, the biomass fraction of municipal solid waste, when used as a fuel for energy production, is recognised as a renewable energy source and is therefore eligible for government subsidy under Estonian legislation. Also CO2 emissions are accounted for according to their source, either as biogenic (resulting from the biomass fraction of the waste), and thus climate neutral, or as fossil, and thus liable for CO2 charges. Therefore, it is crucial that operators of waste incineration plants are able to determine the ratio of biogenic and fossil carbon in combustible waste. This paper summarises the results of the research conducted in the first waste incineration plant in Estonia and the Baltic States to determine the material composition (incl. biomass content) and the proportion of fossil CO2 emission from combusted mixed municipal solid waste. The results of a one year long manual waste sorting study and laboratory analysis show that the share of the biomass fraction is approximately 52 % in the combusted municipal solid waste (on a dry weight basis) and that fossil carbon accounts for about 40 % of the total carbon (on a wet weight bases). The associated annual average fossil CO2 emissions are about 429 kg per ton of combusted municipal waste.