This work reports the thermogravimetric characterization of the solid products resulting from the low temperature reaction between SO2 and mixed solids containing hydrated mixtures of calcium hydroxide and fly-ashes. The latter sorbents were obtained after a pressurized hydration procedure of coal fly-ashes and commercial calcium hydroxide following a factorial design of the experiments. The analysis of the TG curves performed in a high temperature thermobalance allowed calculation of yield of the desulfurization reaction expressed as mol SO2/mol Ca, a parameter needed for the design and optimization of desulfurization processes.