Excessively hard filter cakes have occurred during filtration of fine particles from hot gases in pressurized fluidized bed combustion (PFBC) power plants in which dolomitic limestone was used as the SOx sorbent. These deposits forced premature plant shutdown and became a major barrier to successful development of the filtration technology. Our previous work implicated Mg2Ca(SO4)(3) as a chief cause of these excessive cake strengths, but no analytical technique was available to quantitatively relate filter-cake physical properties and operating problems to Mg2Ca(SO4)(3) concentrations. Hence, a new Fourier transform infrared spectroscopy technique has been developed to analyze for Mg2Ca(SO4)(3) in the presence of CaSO4 and other compounds in PFBC samples. This technique was used to measure chemical compositions of various samples taken from either the cyclone or the hot-gas filtration vessel of a PFBC demonstration power plant. These measurements clarify and further demonstrate the critical role played by Mg2Ca(SO4)(3). Further measurements of the type described here should allow researchers to quantitatively determine how Mg2Ca(SO4)(3) concentrations in filter cakes depend on various operating parameters (e.g., temperatures, pressures, and composition of the coal and sorbent), and thus solve problems that have prevented the successful development of hot-gas filtration for PFBC power plants.