The performance of solid-gas reaction heat transformer system using MnCl2 and CaCl2 was investigated with the consideration of multistep reactions between CaCl2 and NH3. The reactions between CaCl2 and NH3 could be CaCl2.2/4NH(3) and CaCl2.4/8NH(3). The simulated results were verified by the experimental data. From the analysis results, it was concluded that the two reactions between CaCl2 and NH3 existed simultaneously. The favored conditions for the simultaneous occurrence of multistep reactions were discussed. The main reaction in the system with the initial state of CaCl2.2NH(3) and CaCl2.4NH(3) were designated as the reaction of CaCl2.2/4NH(3) and CaCl2.4/8NH(3), respectively. It was concluded that high driving temperature T-M, large relative gas volume, and large specific heat transfer area were the favored conditions for the simultaneous occurrence of multistep reactions in the system both with the initial state of CaCl2.2NH(3) and CaCl2.4NH(3). High initial charging pressure P-0 was favored for the system with the initial state of CaCl2.2NH(3), while low P-0 was favored for the system with the initial state of CaCl2.4NH(3). The impacts of the simultaneous occurrence of multistep reactions on the system performance indicators [temperature lift, specific power, and system coefficient of performance (COP)] were also investigated in this article. It was concluded that the occurrence of the reaction of CaCl2.4/8NH(3) for the initial state of CaCl2.2NH(3) led to better system performance, i.e., larger temperature lift. larger specific power. and larger system COP. However, the occurrence of the reaction of CaCl2.2/4NH(3) for the initial state of CaCl2.4NH(3) would lead to lower specific power. The temperature lift and system COP were larger for the initial state of CaCl2.4NH(3): while the cycle period was shorter for the initial state of CaCl2.2NH(3). (C) 2008 American Institute of Chemical Engineers.