The dry reaction between SO2and limestone has been investigated at low temperatures. The study was focused on the wet-dry scrubbing application. Parameters investigated included: temperature: 313–353 K, SO2concentration: 50–4000 ppm, oxygen concentration: 0–9 percent, carbon dioxide concentration: 0–10 percent, relative humidity 0–92 percent, limestone panicle diameter: 4–100 microns, and limestone conversion: 0–95 percent. The study has revealed that the relative humidity, the particle diameter and the limestone conversion have the most dramatic impacts on the reaction rate. A suggested reaction mechanism is outlined in great detail.