Abstract The solid combustion residues generated during fluidized bed coal combustion and pulverized coal differ as a result of different operating conditions. This paper summarizes a study carried out on the combustion residues arising from two types of fluidized bed combustors, both pressurized bubbling and atmospheric circulating (Escatron power plant and Ciemat pilot plant, respectively), and a pulverized coal power plant. The influence of the combustion system on the characteristics of the residues and the problems related to their disposal, from the point of view of leaching characteristics, were studied. The results show that the main components of the solid residues from fluidized bed coal combustion are those arising from the sulphation reaction and by-products of this reaction, while the inorganic constituents of coal are the main components from pulverized coal combustion residues. Resulting from this composition the most important components, as regards to concentration, are calcium and sulphate in the leachates from fluidized bed combustion residues, while the leachates from pulverised coal combustion residues contain only the elements from the mineral matter. One other interesting observation is the high concentration of free-lime in circulating fluidized bed combustion residues as compare to the pressurized bed combustion residues.