In the framework of the UN Clean Development Mechanism, 12 composting plants were constructed in Uganda. As separate waste collection at the source is not in place, these composting plants receive unsorted, mixed municipal solid waste. This leads to relevant amounts of composting residuals, which visually appear to contain a high share of plastics and are currently dumped into the areas surrounding the plants or landfilled. At the same time, the utilization of alternative fuels in energy-intensive industries, such as the cement industry, is not yet well-established in Uganda (in some European countries more than 80% of fossil fuels are substituted in cement plants). Currently, the predominantly used fuel in Uganda’s cement industry is coal, hence, the utilization of the rejects from the composting plants would not only alleviate environmental burdens caused by waste dumping, but the substitution of coal with refuse-derived fuels (RDF) could also lead to reduced fossil CO2 emissions of the cement industry. The goal of this study is to evaluate the potential of composting rejects of two composting plants in Uganda to be used for recycling purposes or as RDF in cement plants. Reject samples are collected and characterized in detail regarding their fractional compositions, heating values, and their respective chlorine and heavy metal contents. Additionally, the fossil and biogenic shares of the samples are analysed in order to determine specific fossil CO2-emission factors. To differentiate between fossil and biogenic fractions in the samples, a novel, TU Wien-developed method is applied and adapted in order to fit to the Ugandan context. Results of the manual sorting show that the main fractions in the composting rejects are plastics (30-40%) and organics (30-45%). A potential recyclable fraction of 21 to 53% was preliminarily determined. The share of high calorific fractions was found to be between 45 and 55%.