Ammonium ions (NH4+) were intercalated into the lattice of octacalcium phosphate (OCP) via a hydrolysis reaction in a concentrated NH4+-PO43--H+ solution at a low temperature. The NH4+ ions substituted for Ca2+ ions conjugated to P5-PO4. The formation of OCP-NH4 was indicated by an extra peak at 4.3 degrees in the X-ray diffraction pattern.