The injection of calcium-based sorbents into coal-fired boilers for reaction with, and reduction in the levels of, sulfur dioxide (SO2) in the flue gas has undergone considerable research and development. Significant effort has also been made in developing models for the overall reaction CaO + SO2 + 1/2 O2 <-> CaSO4 in order to better predict the effects of system and sorbent variables upon performance. The article discusses the XRD-determination of the identity of CaSO4, formed by the reaction between CaO and SO2, and the support of that determination by density measurements with helium pycnometry. The anhydrous CaSO4 compound formed has an orthorhombic crystal structure and an analytically calculated density of 2.98 g/cu cm, versus the JCPDS reported value of 2.995 g/cu cm. The product of the reaction, formed during calcium-based sorbent injection for SO2 removal from coal-fired power plants, has largely been assumed to have a density of 2.61 g/cu cm. Since reaction models must incorporate the product density in determining the rate of sorbent pore blockage or filling, correct identification of the reaction product is crucial.