The paper discusses recent investigations that indicate that a small addition of certain chemical reagents can enhance or inhibit the ability of calcium sorbents to capture sulfur. The need to develop a low-cost retrofittable SOx control technology for existing non-NSPS boilers has caused renewed interest in the injection of calcium-based sorbents; however, the removal of SOx at a calcium/sulfur ratio of 2 has been limited to about 50%. Initially, Cr203 was found to dramatically improve the sulfur capture of limestone. Chromium (e.g., Cr203) combined with calcium-based sorbents increased sulfur capture by a factor of 1.5-3 at conditions simulating a coal-fired boiler. Subsequently, alkali metal compounds (Li, Na, K) gave similar results. While small amounts of alkali metals enhanced sulfur capture by the CaO, they were also found to react with SO2 to form alkali metal sulfates. Some refractory oxides and clays typically found in coals were shown to reduce the effectiveness of additives which improve sulfur capture. While calcium utilization for the advanced sorbents has increased capture efficiencies with the use of promoters, the mechanisms of promotion have not been identified. The investigation studied mechanistic processes by which additives promote the ability of sorbents to capture SO2 in coal-fired boilers.