Limestone dissolution is a very important factor in flue gas desulfurization systems because it determines its reactivity toward SO2. Fly ash, a siliceous material, has been reported to improve sorbent reactivity. This study investigates the effect Of adding fly ash to limestone on its dissolution rate constant. The experiments were carried out using a pH stat apparatus where the effects of the reaction variables, fly ash/limestone ratio, slurry pH, reaction temperature, and concentration of acid, used were investigated. The central composite design (CCD) of the experiment was used to develop a model that correlates the dissolution rate constant and the reaction variables. It was found that fly ash had a positive effect on the dissolution rate constant of limestone, with the pH having the most significant effect. The dissolution rate constant was found to increase with an increase in the temperature and acid concentration. X-ray diffraction (XRD) analysis showed products of hydration formed, which are mainly calcium silicate hydrates, on the samples This led to an increase in the specific surface area, as observed in the Brunauer-Emmett-Teller (BET) analysis.