The put-pose of this study is to compare the Suitability of five laboratory methods for predicting the ash sintering occurring during the combustion of biomass. The Studied methods have been the following: fusibility under DIN norms, measurement of the compression strength of ash pellet previously heated at different temperatures, manual disintegration of biomass ash obtained at different temperatures, relation alkaline earth oxides to alkaline oxides, and representation in a ternary phase diagram SiO2-CaO-K2O. The two last theoretical methods are based on the analysis of biomass ash elements. The ash sintering behaviour results obtained with these methods for different biomasses have been compared with the ash sintering results obtained in combustion tests of the same biomasses in a 1 MWth bubbling fluidised bed combustion (BFBC) pilot plant, with silica as initial bed material. The compression method does not predict adequately the ash sintering behaviour Of woody fuels and other biomasses with a lower content in alkaline elements. The disintegration and fusibility methods predict with success the sintering behaviour of most of the tested biomasses. Both theoretical methods have been less reliable to predict the ash sintering behaviour than disintegration and fusibility test methods. (c) 2005 Elsevier Ltd. All rights reserved.