The absorption efficiency of gas desulfiarization in a wet-desulfurization system was experimentally investigated using magnesite slurry (main component, magnesium carbonate). Continuous monitoring of a pilot plant for 4 months indicated that magnesite could be used as desulfurization agent when it was ground to a diameter of less than 12 mum. Operational problems did not arise in a pilot plant. Using a magnesite mineral with an average grain diameter 12 mum, a desulfurization ratio equal to or greater than 95% was achieved for a factory waste gas. The properties of magnesite were modified in order to improve the reaction ratio. It was determined that heating improved the reaction ratio. This observation can be incorporated in designing new desulfurization equipment.