Composite particles of calcium chloride and expanded graphite were prepared for promoting the reaction of gas-solid chemical heat pumps that generate cold heat by using waste heat or solar energy. Analyses with SEM and mercury porosimetry revealed that fine particles had been created in small pores of the expanded graphite powder by a procedure developed in this study. TG analysis for the reaction with methanol vapor showed that the reaction rate of composite particles with a large specific surface area was about 1.6 times as large as that of untreated calcium chloride.