Biomass-based waste fuels are used in many industrial applications since combustion of biomass gives no net emissions of carbon dioxide. Some waste fuels, e.g. RDF (refuse derived fuels), contain not only biomass, but also some fossil material, hence can be classified as partially CO2 neutral fuels. The biomass fraction of a mixed solid fuel is an essential parameter for the determination of net CO2 emissions. It is also important to know the accuracy of the different biomass fraction determination methods. In the present study, the biomass fraction of artificially made RDF was determined by means of the SDM (selective dissolution method) in total carbon basis and also by the C-14 method in total carbon, weight and calorific value bases. Then the relative accuracy was compared using findings from a previous study. SDM shows very high accuracy in weight and calorific value basis. The C-14 method gives comparable results in most cases, but shows considerable deviations for some samples. The SDM results in total carbon basis show less accuracy compared to the other bases. When performing the SDM analysis in total carbon basis, one should correct for ash forming matter in order to increase the accuracy of the biomass fraction determination. (C) 2014 Elsevier Ltd. All rights reserved.