An experimental system was built to test the effect of various process parameters on the performance of the Modified Dry Limestone Process (MDLP) for flue gas desulfurization. Two types of limestone, one calcitic and one dolomitic, were used. These materials were characterized by ICP analysis, X-ray diffraction, optical microscopy, SEM, and electron microprobe before and after reaction. Performance was judged on the basis of the formation of a friable gypsum reaction product and the maintenance of a pH of about 4.84 or higher in water through which the exit gases were bubbled. Two primary and one secondary parameter were identified as the most important for optimum performance of the MDLP. The two primary parameters were temperature and water content. A temperature of 68°-70°C promoted reaction, while no reaction occurred at 31°C. The solubility of SO₂ in water was the controlling factor for water content. A maximum ratio of about 3.4 g SO₂/100 g water at 69°C was necessary. The secondary parameter was the type of limestone used. A dolomitic limestone with a reasonable amount of Fe performed better than either marble or a calcitic limestone, both low in Fe. A reasonable amount of Fe and an extensive pore structure seem to be the most important factors in limestone SO₂ absorption performance. / Master of Science