Coal-fired boilers often develop massive fouling and slagging deposits on heat transfer surfaces in areas where coal ash particles have relatively high viscosities (>= 10(6) Pa s). In order to investigate the characteristics and mechanism of the problems, experiments were performed in a drop tube furnace and by use of a slagging probe in this study. The results showed that fouling deposits developed on the probe in the first stage, and then transformed into partially or fully fused slagging deposits. The deposition of high-viscosity coal ash was closely related to both the experimental conditions and the chemical composition of the ash. A low temperature on the probe surface and a high impact velocity of ash particles significantly inhibited ash deposition for the high-viscosity ash. The relative deposition ratio of high-viscosity ash, defined as the ratio of the mass of the deposit on the probe to the total mass of ash fed into the DTF, increased along with time initially and then diminished after reaching the maximum. Fine ash particles had higher deposition tendency compared to coarse particles, probably due to local eddies between the fingers on the front of the probe. The chemical compositions of coal ashes had an observable effect on the deposition behavior of high-viscosity ash. The findings of this work are significant for understanding the mechanism of high-viscosity ash deposition and determining the effect of key operating conditions relevant to deposition of high-viscosity ash. (C) 2013 Elsevier Ltd. All rights reserved.