The effect of inorganic constituents on the thermal behaviour of low rank coals, as well as agricultural and forestry by-products, during pyrolysis and combustion processes was investigated. Selective demineralization of each fuel with acids was used. Raw and demineralized samples were analyzed for ash content and composition (mineralogical and chemical). surface area and porosity. as well as thermal conversion characteristics in inert and air atmospheres. Reactivity and kinetic experiments were performed by non-isothermal thermogravimetry, over the temperature range of 25-850 degrees C, at a heating rate of 10 degrees C/min and with a material particle size of -250 mu m. The extent of mineral matter removal from the materials studied, ash composition and structural characteristics depended upon the type of the fuel and the method of acid treatment. Generally, calcium, magnesium, potassium and silicon minerals acted as inert materials. inhibiting the pyrolysis and combustion rates of the samples. However, they increased the sensitivity of the reactions. by lowering the peak temperatures. A first-order parallel reactions model for pyrolysis and a power low model for combustion fitted the experimental results accurately. Mineral matter had a small influence on the kinetic parameters of biomass samples, but appeared to hinder the overall process in the case of the coals. (c) 2006 Elsevier Ltd. All rights reserved.