Four samples of fly ash from thermal waste conversion were analyzed for the content of heavy metals (Zn, Cu, Pb, Ni, Cr, Cd, As, V, Tl and Hg) and oxides of selected elements (Si, Fe, Al, Mn, Ti, Ca, Mg, S, P, Na, K, Ba and Sr). The fly ashes showed a high content of CaO (14.2-35.4%) and SiO2 (15.4-49.2%), which classified them as an additive to concrete, while the high content of leachable chlorides (27.8-38.8 g/kg) and sulfates (384-955 mg/kg) limited their use.