The slagging behaviour of 12 different coals/blends was analysed in an entrained flow reactor and compared with predictions using a wide range of empirical indices. Large discrepancies were obtained in most cases, both with respect to experimental results (in terms of capture efficiency and energy-based growth rate) and among the different indices. A new approach is proposed, in which the traditional expressions are modified to account for other parameters with influence on deposition phenomena: aerodynamic diameter and the total mass of mineral matter injected with the coal. Each of these factors resulted in improved correlations between predictions and experimental observations for all the indices analysed, which together with the good agreements achieved with several of the new indices are thought to confirm the validity of the approach proposed. (c) 2007 Elsevier B.V. All rights reserved.