The scope of this paper includes the development of a modelling approach to predict the ash release behaviour and chemical composition of inorganics during co-firing of coal and biomass. In the present work, an advanced analytical method was developed and introduced to determine the speciation of biomass using pH extraction analysis. Biomass samples considered for the study include wood chips, wood bark and straw. The speciation data was used as an input to the chemical speciation model to predict the behaviour and release of ash. It was found that the main gaseous species formed during the combustion of biomass are KCl, NaCl, K2SO4 and Na2SO4. Calculations of gas-to-particle formation were also carried out to determine the chemical composition of coal and biomass during cooling which takes place in the boiler. It was found that the heterogeneous condensation occurring on heat exchange surfaces of boilers is much more than homogeneous condensation. Preliminary studies of interaction between coal and biomass during ash formation process showed that Al. Si and S elements in coal may have a 'buffering' effect on biomass alkali metals, thus reducing the release of alkali-gases which act as precursors to ash deposition and corrosion during co-firing. The results obtained in this work are considered to be valuable and form the basis for accurately determining the ash deposition during co-firing. (C) 2009 Elsevier B.V. All rights reserved.