The alterations of ash quality and utilisation aspects when co-firing coal with biomass were investigated. Co-combustion tests were performed in lab and semi-industrial scale facilities, using several coal-biomass blends. The collected ash samples were analysed for major elements and heavy metals content, loss on ignition (LOI), free CaO content and grain size distribution. Since a variety of co-combustion residues were tested, important implications concerning the ash composition and, consequently, its further use in potential applications came up. Results showed that properties of co-combustion residues are directly connected to the combustion conditions and the individual blend components. Biomass exploitation as secondary fuel in co-combustion processes is technically and economically feasible up to 20% w/w and the produced ash could be further utilised without any major treatment. (c) 2006 Elsevier Ltd. All rights reserved.