Fluidized bed combustors (FBCs) are noted for their ability to capture SO2 in situ via direct reaction with Ca-based sorbents. However, despite more than 30 years of intensive study of sulfation processes in atmospheric FBC boilers and numerous laboratory studies, there are still many uncertainties and disagreements on the subject. In particular, the mechanisms of the sulfation reaction are still not properly understood, and there is dispute over the explanation of the well-known temperature maximum for optimum sulfur capture found in FBC boilers. This paper discusses these points of contention and suggests the most probable mechanisms and explanations for the various phenomena seen with sulfur capture, based on current literature and personal experimentation. (C) 2001 Elsevier Science Ltd. All rights reserved.