Deposits formation on boiler heat transfer surfaces is one of the main operational problems associated to biomass co-combustion. This paper proposes a novel method to rank deposition propensity of biomass fuels. The method combines thermo-mechanical analysis (TMA) with ash leaching procedures and is applied here to a set of biomass fuels. Based on the TMA penetration and shrinkage traces obtained for the original and the leached ashes, the fuels are classified regarding their deposition tendencies. The results have been successfully validated with chemical analysis of the liquid fraction from ash leaching. Initial results for the behaviour of coal-biomass mixtures are also presented. (C) 2007 Elsevier Ltd. All rights reserved.