Gasification is playing an increasingly significant role in thermal processing. The different types of solid waste generated in these processes should ultimately be treated as by-products and be used in various industries (such as fly ash from coal combustion, for example). However, studies of waste properties are needed to determine real and potential impact on the environment. The scope of our tests depends on methods and directions of planned use of waste. Our paper presents and discusses the test results of waste generated in research installations: one in a large laboratory and the second in a pilot plant. The wastes are derived from the process of direct combustion, direct gasification, and the two-stage process of coal gasification with char formation followed by combustion. All installations are based on the fluidized bed, with two hard coals and lignite used as fuel. Tested materials belong to two groups: fly ash and bottom ash. Research of the waste properties was conducted in the field of grain size distribution, chemical composition, content of combustible leaching of chemical contaminants, and radioactivity. Analysis of the results was carried out both in terms of existing legislation and also related to a comparison of properties of materials from single-stage and two-stage thermal processing.