Municipal solid waste (MSW) is considered a valuable energy source that can be applied as a secondary (co-combustion) or even primary fuel in a power plant. Unfortunately, due to some formal (legislative) and technological barriers, MSW is not widely applied in the Polish energy sector. Especially, a crucial problem is the utilization of part of MSW called the over-sieve fraction (OSF). This fraction, which accounts for 30%-40% of MSW, is a by-product from the mechanical separation of MSW. Crude OSF can generate problems because it includes moisture and organic matter. Thus, it is not a stable material and can generate some technological and hygienic problems due to suppuration, unpleasant odor, and parameter changes during storage. These problems can be solved by the application of low-temperature pre-treatment of solid waste. During this process, some physical and chemical properties of crude OSF can be improved, such as calorific value, energy demand for fragmentation and milling, and property stability. The aim of this paper is to investigate the physical and chemical properties of OSF after low-temperature (150-250 degrees C) treatment. Proximate and ultimate analysis of waste after thermal pre-treatment are presented and discussed in terms of waste application as an alternative fuel in the energy sector.