An increasing number of municipal sewage treatment plants in Poland, desirable from an environmentalperspective, raises the problem of managing the growing volume of sewage sludge. The thermal treatment ofmunicipal sewage sludge (TTMSS) method, by greatly reducing the waste volume, increases the heavy metalconcentration in fly ash (primary, end product of the treatment process), which may constitute a risk factor whenattempting to utilize them economically. The research paper concentrates on determining the TTMSS fly ash heavymetal leaching level. For this purpose, ash samples were subjected to leaching with the batch and percolation tests,and the heavy metal content in eluates was determined by the FAAS method. The obtained results served as a base to determine the level of heavy metal immobilization in the ash, the element release mechanism (percolation test),and the impact of the L/S (liquid to solid) ratio and pH on the heavy metal leaching intensity (percolation test).The conducted research indicated high immobilization of heavy metals in TTMSS fly ash, regardless of the appliedstudy method, which corresponds to the results of other researchers. Lead was the most intensively eluted metal.