Semi-batch precipitation of calcium monohydrogen phosphate dihydrate (CaHPO4·2H2O; also termed dicalcium phosphate dihydrate, DCPD) from a homogeneous reaction of KH2PO4 and CaCl2 was studied with inline Raman spectroscopy. In the reaction, hydroxyapatite (HAP, Ca5OH(PO4)3) formed first and transferred to DCPD throughout the entire precipitation process. Raman calibration models were created with a series of KH2PO4-NaOH-water solutions. The reliability of the calibration models was validated with offline measurement of phosphate concentration in the mother liquor. Real-time changes in HPO4 2- and H2PO4 - concentration were monitored and predicted. Comprehensive kinetic information was obtained from liquid and solid analysis. It was found that the influence of the mixing rate on the precipitation was not significant, whereas the reactant feeding rate had a remarkable impact.