Particulates from the combustor bed drain, cyclone drain and filter cakes of the Tidd 70 MWe 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 CaSO4 and MgO. The elutriated particles captured by the cyclone and filters also contained ash and CaSO4, but X-ray diffraction patterns revealed that the Mg in the filter cakes was present as a compound known in the literature as Mg3Ca(SO4)(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 SO2 emissions by the filter vessel. However, it was also shown that the correct stoichiometry of this important compound is Mg2Ca(SO4)(3), not Mg3Ca(SO4)(4).