The aim of this work is to develop and test a method for assessing the size and the morphology of biomass fuels and their chars. Severe thermal conditions (high temperature and heating rate) are programmed during the pyrolysis, focusing on characteristics of chars in combustion or gasification. An image analysis program is used to quantitatively study several scanning electron microscopy images of fuel and char samples. Distributions of results are obtained for a significant number of particles from a statistical point of view. Average values and the standard deviations of the distributions quantify the heterogeneous nature of the fuel and char particles, providing useful parameters for advanced modeling. Size, shape factors (aspect and roundness) and superficial parameters are defined and measured (or calculated) developing a procedure for a low time-consuming analysis. The structural variations caused by the fast release of a high amount of volatile products are evaluated comparing the results obtained on fuel and char particles. The results are discussed to assessing the suitability of the selected parameters and the possibility to quantify softening, melting, shrinking and fragmentation phenomena. The method is applied to two biomass fuels of different origin and chemical composition: wood pellets and olive residue. (C) 2007 Elsevier Ltd. All rights reserved.