Experiments have been done subjecting ashes from industrial-scale FBC boilers to sulphating conditions in an oven for up to 105 days. These show that sulphation by itself causes agglomeration in the virtual absence of V, K, and Na, the elements normally associated with ash softening and classical fouling. In addition, it has been demonstrated that sulphation goes to completion over long periods of time and, at a specific level which differs from one ash to another results in agglomeration. These experiments have also shown that there is a size range (75-300 mu m) in which the agglomeration is worst, and particles that are smaller or larger either do not agglomerate or agglomerate more weakly. Added "inert" coal-derived ash decreases or prevents the agglomeration. However this ash does not appear to chemically combine with the sulphate, but acts by mechanically separating the sulphating particles. Finally, if alkali metals are present they can cause agglomeration at levels lower than those at which either the alkalis or sulphation separately cause agglomeration, i.e., they operate synergistically to cause fouling. Current work is being directed at examining these phenomena at higher temperatures (900 degrees C and above).