The behaviour of the mineral matter in a fuel may crucially affect the availability of a boiler when the fuel is fired. The ash may cause severe problems in the flue gas channel in forms of fireside deposits on heat exchangers. These deposits lower the efficiency of the boiler and cause in the most severe cases an unscheduled shutdown. In this paper we report results from a study where the ash behaviour was monitored in a pulverized wood fired boiler. Short-term deposit sampling was combined with in situ fly ash and flue gas sampling as well as advanced fuel analyses. By combining these three tools we could track down a chain of events the ash went through from the point where it was introduced into the boiler with the fuel until the stage where it formed a deposit on a heat exchanger tube. Sub-micron sized ash particles found in the flue gas with a Berner-type low-pressure impactor were enriched in alkali, Sulphur and chlorine. Similar particles were also found on the backside of the air-cooled deposit sampling probes, forming thin initial alkali, sulphur and chlorine-rich deposit layer. These elements were further found by advanced fuel analysis to be associated with the moisture or the organic phase of the fuel. Larger ash particles of the size of 1-10 mum found in the flue gas with the low-pressure impactor were found to deposit of the front side of the sampling probe. These particles consisted mainly of calcium, most likely oxide or carbonate. With the advanced fuel analyses we could find these particles already as mineral particles in the wood fuel. We also saw some indication that peat could act as a cleaning fuel. In general the results show that a detailed well-performed fuel analysis is a key knowledge when ash behaviour predictions are to be made. (C) 2004 Elsevier Ltd. All rights reserved.