The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super-heater tubes in a biomass-fired CFD boiler. The deposit layer thickness and the soot-blowing frequency effect on the super-heaters heat transfer are the main subject of the study that has been conducted over a 3-year period. The measurements show a deposit growth rate on the super-heaters of approximately 4gm(-2)h(-1). The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super-heater, the fouling problem on the super-heater and re-heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super-heater by nearly 40%. The soot-blowing system shows a strong positive effect on the heat transfer rate of the super-heater a few hours after a soot-blowing sequence has been completed. However in the long run, the varied soot-blowing frequency does not have a significant influence on the deposit layer growth rate. Copyright (c) 2006 John Wiley & Sons, Ltd.