We have explored the possibility of introducing biologically relevant ions, namely Sr(2+), Mg(2+), and Mn(2+), into the structure of octacalcium phosphate (OCP). OCP with partial substitutions of Sr(2+) and Mg(2+) for Ca(2+) up to 7.4 and 1.0 at%, respectively, was successfully synthesized. On the contrary, Mn(2+) completely inhibits OCP crystallization, and it promotes the precipitation of brushite. The incorporation of Sr(2+) and Mg(2+) disturbs the shape of the crystals and reduces the stability of OCP structure. The analysis of the structural modifications occurring during heat treatment puts into evidence that the transformation of OCP into apatite involves a crystalline phase, "collapsed OCP", resulting from loss of structural water molecules. The main characteristic features of this phase are the small angle reflection at about and 17.0 angstrom (2 theta = 5.2 degrees) and the reflection at 8.6 angstrom (2 theta = 10.3 degrees). Due to the destabilizing effect of the foreign ions, in the temperature range of 170-250 degrees C, Mg- and Sr-OCP contain "collapsed OCP" but no OCP, which allowed us to calculate the unit cell of this new phase characterized by a slightly reduced a-axis and a wider gamma angle with respect to OCP.