CaSO4 and Ca(OH)(2) are the two main soluble phases in ash from the fluidized bed combustor (FBC) combustion of high-sulfur fuels with limestone addition. Their solubilities are of critical importance to several problems ranging from leaching to sorbent reactivation processes with liquid water. To determine these parameters, realistic models are necessary to estimate the activity coefficients of ionic components present. The current results indicate that the decrease of CaSO4 solubility in the presence of Ca(OH)(2) is smaller than suggested in some earlier calculations, using more simplified assumptions. Experimental data from real FBC ashes have been obtained for comparison, and the current model predictions and experimental results are in excellent agreement.