Fluidized bed combustor (FBC) ashes from high-sulfur, low-ash fuels, can agglomerate if subjected to sulfating conditions for long enough (days to weeks). The degree of sulphation increases with both temperature and time under these conditions, and at a conversion equivalent to the production of 50-60% or more of CaSO 4 in the deposit the ashes agglomerate. Fly ash agglomerates less readily than does bed and loop seal ash and produces weaker deposits, although all of these materials will agglomerate if sufficient time is allowed. The potential for agglomeration increases if the temperature is increased from 850 to 950ЊC. Agglomeration also occurs at lower temperatures (down to at least 750ЊC), but the mechanism may be via carbonation and then sulphation of the ash. Although experiments reported here suggest that if pure CaSO 4 is compressed to the 140 kPa range it does show some tendency to agglomerate, the agglomeration of FBC ash is not produced simply by the formation of CaSO 4 . Finally, the agglomeration process is only weakly influenced by the partial pressure of SO 2 in the flue gas. Attempts to identify physical parameters to differentiate the tendency of various bed materials to agglomerate have been only partially successful. Two bed materials with strong and weak agglomerating tendencies were studied. These were shown to have very similar particle shapes and only slightly different angles of repose, but quite different bulk densities. Residues with a greater bulk density appear to have a stronger tendency to agglomerate, and this may provide a method of ranking the agglomeration potential of different bed materials.