The effects of potentially important process variables in a fly ash/hydrated lime sorbent producing process on the reactivity of the sorbents were investigated. The process variables investigated were fly ash/hydrated lime weight ratio, reaction temperature, reaction time and additive concentration. The fly ash/hydrated lime sorbents were produced in a lab-scale batch reactor and the reactivity of the produced sorbents was evaluated in terms of SO2 removal efficiency and sorbent conversion determined in a lab-scale fixed-bed reactor. The experimental conditions in the sorbent producing process were designed following a 24 factorial design method and the effects of the process variables were analyzed by ANOVA technique. The analyses of the experimental results showed that as the fly ash/hydrated lime weight ratio was increased, the SO2 removal efficiency decreased but the sorbent utilization increased due to the decrease of the Ca/S ratio with increasing fly ash/hydrated lime weight ratio. It was also shown that both the SO2 removal efficiency and the sorbent utilization increased with increasing reaction temperature, with increasing reaction time or with increasing additive concentration due to the increased rate of silica dissolution at higher reaction temperature and at higher additive concentration or due to the increased amount of dissoluted silica from fly ash with longer reaction time.