Mixed combustible wastes and refuse-derived fuels (RDF) usually consist of both biogenic and fossil materials. To report climate-relevant CO2-emissions or declare the share of renewable energy recovered in Waste-to-Energy (WtE) plants or in industrial processes, it is necessary to distinguish between the biogenic and fossil origins of the utilized waste components. In recent years different methods have been developed for this purpose. Standardized methods are: Manual Sorting (MS), the Selective Dissolution Method (SDM), the Radiocarbon Method (14C-Method), and the Balance Method (BM). The BM is based on the evaluation of WtE plant operating data and is already applied in several WtE plants in Europe. The BM was further developed into the adapted Balance Method (aBM), which is applicable to waste samples and evaluates data on the elemental composition of the samples. The feasibility of the aBM in practice has, however, not been demonstrated yet. The objective of the present thesis is to assess the BM and the aBM in terms of their feasibility, versatility, and reliability with respect to two major fields of application: (A) Large-scale application of the BM to examine the eligibility of the method and to exploit further applications of BM-generated data. Operating data of 10 Austrian WtE plants are evaluated over a period of one year. Different parameters are derived: i.a. fossil CO2-emission factors (EF), the ratio of biogenic energy from waste (qB) or the plastic content (xplastic). (B) Characterization of artificially prepared RDF model mixtures as well as real RDF samples by means of the aBM. Performance data of the aBM are generated, indicating the validity of the method and providing the basis for standardization. The results related to (A) show that with the aid of the BM, almost 90 % of the waste utilized in Austrian WtE plants annually could be characterized in terms of its composition. This represents a “sample size” which can hardly be achieved by any other method. Generated annual means for the WtE plants as well as monthly mean values at plant-level cover a wide range for all parameters considered. This indicates a strong variability of the waste composition, which is believed to be influenced by varying RDF demand of the cement industry, downtime periods of other WtE plants, or regional differences in waste collection. The observations clearly illustrate that the usage of an empirical value for EF or qB for WtE plants is problematic and can easily lead to significant misestimations of relevant data (by up to 40 % at plant-level). Similarly, it can be assumed that data on the plastic content in mixed waste, generated by sorting campaigns, leads to distorted estimates. Detailed information on the share of plastics in the feed of WtE plants, easily derived by applying the BM, represents valuable information when overall waste plastics generation and its utilization paths have to be assessed. Furthermore, the results reveal that temporally high-resolution data of the waste composition could be used to trace back insufficient mixing of the waste feed. In addition, the mandatory plausibility checks of the BM can support error detection in the operating data of WtE plants. The investigations related to (B) represent the first in-depth assessment on the performance of the aBM. Based on the aBM-determined share of fossil carbon (xTC F ), an absolute deviation from the reference value of below 3:6 % can be asserted. The comparison of aBM-generated values to results of MS and the SDM reveals that the aBM is the only method for which, independently of the RDF, low deviations and good correlations with the 14C-Method are observed. A more extensive sample preparation is expected for the aBM compared to MS and the SDM. This is due to the aBM relying on elemental analysis where only a few centrigrams of sample can be measured. By means of a variance component analysis, it is shown that the drawing of test specimens from the analysis sample considerably affects the total variation of aBM-results. A usage of different grinding mechanisms can be recommended for the sample preparation to ensure a low heterogeneity of the test specimens. A further crucial factor for the application of the aBM is the choice of the necessary input values the elemental composition of the water-and ash-free fossil and biogenic matter present in the RDF (TOXF and TOXB). These values are ideally generated specifically for an RDF by means of initial manual sorting and analyses. Thereby the highest accuracy of the aBM-results can be expected. However, the thesis shows that TOXF and TOXB values derived for 6 different RDFs are in a close range. This indicates that in future workload could be saved once a suitable data base about TOXF and TOXB values present in different types of RDFs is available. Finally, to conclude on the practicability of the aBM in comparison to standardized methods, the aBM presents itself as competitive with MS, the 14C-method and also to the commonly applied SDM in terms of workload and costs. The thesis shows that both the BM and the aBM are practicable methods to determine the fossil and biogenic fraction in mixed waste and RDFs. Both methods are competitive with other standardized determination methods such as the 14C-method, MS, or the SDM. The BM stands out by virtue of its unique capability to deliver data at a high temporal resolution, data which can in future also be exploited for other purposes. The investigations into the performance of the aBM presented clearly illustrate that this method is fit for practical application and can support the formulation of a standard procedure. A transferability of the principle to environmental samples (e.g. mircoplastics content in water samples) is demonstrated and will be the subject of future investigations. Forthcoming research activities should further address the reproducibility of the aBM when applied by different laboratories and should push for a collection of data on the elemental composition of fossil and biogenic matter present in different RDF types.