The origin of the multistep thermal dehydration of calcium hydrogen phosphate dihydrate (dibasic calcium phosphate dihydrate (DCPD)) to form γ-calcium diphosphate (γ-calcium pyrophosphate (γ-CPP)) via calcium hydrogen phosphate anhydride (dibasic calcium phosphate anhydride (DCPA)) was investigated from a specific viewpoint of physico-geometrical constraints generated during the reaction. The overall thermal dehydration was separated into five partially overlapping steps through systematic kinetic analysis. The first three steps and the residual two steps were attributed to the thermal dehydration of DCPD to form DCPA and of DCPA to form γ-CPP, respectively. The first to third steps were kinetically characterized by the surface reaction of plate-like particles controlled by nucleation and growth, the movement of the reaction interface inward to the plate by releasing water vapor through voids formed in the surface product layer, and the rapid escape of water vapor accompanied by the cleavage of plate-like particles into slices, respectively. The contributions of each component step varied with the heating conditions and atmospheric water vapor pressure. The subsequent dehydration of DCPA proceeded in two steps by the release of trapped water molecules in amorphous DCPA induced by its gradual crystallization and the dehydration of DCPA to form poorly crystalline γ-CPP, which continued to grow during the fifth mass loss step and exhibited a detectable exothermic phenomenon after the mass loss was completed. The possible causes of the variation in the multistep reaction features with reaction conditions were discussed by correlating the kinetic analysis results with the crystallographic and morphological findings.