Particulates from the combustor bed drain, cyclone drain and filter cakes of the Tidd 70 MW e pressurized fluidized bed combustion plant were analysed for particle size distribution and for compounds derived from the dolomitic sulfur sorbent injected into the bed. As expected, the bed material consisted chiefly of coal ash, orthorhombic CaSO 4 and MgO. The elutriated particles captured by the cyclone and filters also contained ash and CaSO 4 , but X-ray diffraction patterns revealed that the Mg in the filter cakes was present as a compound known in the literature as Mg 3 Ca(SO 4 ) 4 . Analyses showed that the compound in question was formed in the filter cakes, yielded its production rate, and showed that it was the product of the (unintended) reduction of SO 2 emissions by the filter vessel. However, it was also shown that the correct stoichiometry of this important compound is Mg 2 Ca(SO 4 ) 3 , not Mg 3 Ca(SO 4 ) 4 .