Shear strengths were measured (at 22 degrees C) for powders of MgSO4, CaSO4, and their mixtures both before and after heating at 750 degrees C for 1 h, The strengths were significantly greater for the heated samples and for samples that contained MgSO4. The compound Mg2Ca(SO4)(3) was detected in the heated mixtures. The experiments were motivated by the hypothesis that excessively hard and strong deposits observed in the hot-gas filtration vessel of a first-generation pressurized fluidized bed combustion (PFBC) system were formed by solid-state diffusion and formation of "necks" at points of contact between MgSO4 and/or CaSO4 particles. The experimental results support this hypothesis and predict which limestone/dolomite minerals should be chosen as SOx sorbents.