Abstract <p>A low-temperature synthesis method for calcium phosphate compounds—specifically, octacalcium phosphate (OCP) doped with magnesium, strontium, and barium cations at substitution degrees ranging up to 20 mol %—was developed in this study. The conditions for the phase transformation of α-tricalcium phosphate (α-TCP) into dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O), followed by its hydrolysis to octacalcium phosphate (OCP, Ca₈H₂(PO₄)₆·5H₂O) in buffer solutions (sodium acetate, L-glutamic acid) at near-physiological temperatures (35–40°C), were established. Doping with strontium, barium, and magnesium cations (Sr²⁺, Ba²⁺, and Mg²⁺) was performed during the transformation stages α-TCP → DCPD and DCPD → OCP by adding the corresponding metal nitrates to the reaction media. It was found that strontium cations incorporate most effectively into the OCP structure, while barium cation introduction (above 10 mol %) leads to the formation of impurity phases (barium hydrogen phosphate) and reduced crystallinity. Doping with magnesium cations inhibits phase transformations, resulting in highly defective crystalline structures at concentrations up to 10 mol %. The phase composition, structure, morphology, and chemical composition of the synthesized powders were characterized using X‑ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy, and X‑ray fluorescence analysis. The developed low-temperature method enables the controlled synthesis of cation-substituted calcium phosphates, which is promising for the creation of bioactive materials for bone tissue regeneration.</p>