The present study is focused on the in vitro investigation of octacalcium phosphate (OCP) transformations when mixed with physiological solution (aqueous isotonic 0.9% NaCl) by the energy dispersive X-ray diffraction technique. This technique allowed in situ monitoring of the processes taking place in this system in real time. It was demonstrated that when OCP is mixed with physiological solution, it undergoes two simultaneous processes: (1) amorphous-into-crystalline OCP transformation (37%), and OCP average grain size growth from 22 up to 35 nm; (2) partial phase transformation of OCP (33%) into hydroxyapatite (HA), and HA average grain growth up to 5 nm. Both processes take place simultaneously with the same characteristic time (2.90 +/- 0.25) h. (C) 2010 Elsevier B.V. All rights reserved.