The influence of calcium sulfate (CaSO4) on the physicochemical properties and desulfurization behavior of mixtures of fly ash and calcium hydroxide (fly ash/Ca(OH)(2)) at different initial ratios and hydration times was investigated. CaSO4 decreases the hydration reaction rate, and its influence is less pronounced after 15 h of hydration. Sorbents with smaller particle sizes, higher porosities and specific surface areas, and different compositions are obtained at high hydration times when CaSO4 is present. In relation to the sorbents obtained, CaSO4 has a positive or negative effect on the desulfurization ability, depending on the preparation conditions. The sorbent that is prepared at a higher fly ash/Ca(OH)(2) ratio, hydration time, and gypsum amount (fly ash/Ca(OH)(2)/gypsum ratio of 5/3/1.5 at 37 h) presents the best calcium utilization in the desulfarization. reaction. From the results obtained, it seems that the positive effect on the desulfurization ability for sorbents prepared when CaSO4 is present is dependent on the different silicates with higher surface area formed. This effect is found at shorter hydration times as both the fly ash/Ca(OH)(2) ratio and the CaSO4 amount increase. High fly asb/Ca(OH)(2) ratios and high gypsum amounts will be the experimental conditions used to optimize the preparation of the sorbents.