The mechanisms of HCl reaction with dry Ca(OH)(2) or CaO sorbents in flue gas cleaning applications were investigated by differential scanning calorimetry (DSC), thermogravimetry, and X-ray diffraction, A short-time differential reactor (STDR) was used to contact 1000 ppm HCl with dry sorbents at temperatures ranging from 100 to 600 degrees C. The product of HCl reaction with Ca(OH)(2) over the 100-600 degrees C range in the STDR was identified as a solid solution of CaCl2.2H(2)O and CaClOH. Subsequently, the Cl/Ca molar ratio in the product was determined to be less than 2. The product of the HCl reaction with CaO in the STDR over the 100-600 degrees C range was identified as CaClOH. When scanned by DSC, samples of both CaO and Ca(OH)(2) that were previously exposed to HCl in the STDR revealed an endotherm between 530 and 590 degrees C. The amount of enthalpy correlated well with the chemically determined sample conversion.