Coal-biomass mixtures have been prepared and combusted on the Imperial College entrained flow reactor (EFR). Five different biomasses were mixed with two bituminous coals at levels of up to 60 wt.%. Samples of deposit have been collected and characterised. Higher levels of biomass replacement have increased both the deposition efficiency and the degree of sintering of the ash deposits. The relative rankings of the five biomasses in terms of their impact on these two aspects of deposit behaviour are reversed, in line with previous observations at lower levels of coal replacement by biomass. The relative effects of biomass addition on the degree of deposit sintering can be predicted approximately by calculating fuel ash viscosities from ash coal and biomass chemistries and ash contents. Differences between the observed deposit chemical compositions and those calculated from high temperature ash analyses suggest that volatilisation and incomplete condensation has affected the concentrations of K2O and P2O5 in the deposits. Three-way mixtures of coal, biomass and coal washery fines, a high-ash fuel, have also been studied; the inclusion of coal washery fines has not counter-acted the changes in ash behaviour introduced by the addition of biomass, and would not be recommended for power station operation. (C) 2007 Elsevier B.V. All rights reserved.