Previous studies have indicated that synthetically-precipitated amorphous calcium phosphate, even though not a periodically regular structure, possesses a chemically definable local unit of structure. The present paper is a report of thermochemical studies performed to more clearly define this local chemical unit. Upon ignition in the absence of water, freezedried amorphous calcium phosphate preparations converted into crystalline α-and/or β-tricalcium phosphates. At temperatures just below the crystallization point, the amorphous calcium phosphate became almost completely dehydrated. Additionally, pyrophosphate production at these subcrystallization temperatures was very low. From these data, it appears that water in amorphous calcium phosphate does not exist in combination with the P2O5 component as acid phosphate but retains its molecular identity, suggesting, together with previously reported chemical data, that amorphous calcium phosphate when formed under specific chemical conditions is a hydrated tricalcium phosphate.