Magnesium and strontium both play important roles in the growth of bone and so are desirable ions for substitution into hydroxyapatite (HA) intended for use as bioinstructive bone substitutes. A range of compositions were prepared by a solid state method based on the nominal composition of HA (Ca 10(PO4)6(OH)2), with various levels of strontium and/or magnesium substitution: strontium-substituted HA (Ca 8Sr2(PO4)6(OH)2), magnesium-substituted HA (Ca9.8Mg0.2(PO4) 6(OH)2 and Ca9Mg(PO4)6(OH) 2), and strontium and magnesium co-substituted HA (Ca 7.8Sr2Mg0.2(PO4)6(OH) 2 and Ca7Sr2Mg(PO4) 6(OH)2). Materials were characterised by powder X-ray diffraction, Fourier-transform infrared spectroscopy and Raman spectroscopy. These analyses indicated that the co-substituted materials were composed of mixtures of strontium-substituted hydroxyapatite and magnesium and strontium co-substituted β-tricalcium phosphate. In the magnesium-substituted materials, increased magnesium content was related to increased proportion of β-tricalcium phosphate phase, both with and without strontium co-substitution. The unsubstituted and strontium mono-substituted materials, however, were pure apatite phase, suggesting that magnesium was the destabilising factor in the phase compositions of the magnesium mono-substituted and magnesium and strontium co-substituted materials.