An entrained flow reactor has been used to characterize the slagging behaviour of twelve coals/blends, under different experimental conditions. Besides measuring the weight of the deposits, a detailed characterization of the coals, fly ashes and deposits has been performed using ASTM procedures and Computer-Controlled SEM. Two different indices are used to represent the deposition behaviour: capture efficiency (CE) is a measure of the intrinsic tendency of the particles to form a deposit, while the energy-based growth rate (GRE) also considers the ash and energy contents of the different fuels. The coals tested ranged from lignites to anthracites and showed widely different values for those stagging indices. The experimental results are interpreted making use of a detailed analysis of the physico-chemical properties of the coals and fly ashes. In particular, a consistent relationship is found between deposit growth rates and the aerodynamic diameter of the fly ash particles. Coal blending showed in all cases non-linear behaviours with respect to the parent coals. The experimental programme was also extended to evaluate the influence of variations in some experimental parameters, including exposure time, gas temperature, surface temperature and distance from injection. The results obtained are useful both to characterize the experimental method and to determine the influence of some parameters relevant to deposition phenomena. (C) 2006 Elsevier B.V. All rights reserved.