Four samples of composition Ca10(PO4)6−x(SiO4)x(OH)2−x, with x = 0.0, 0.1, 0.2 and 0.5, were prepared and characterized using powder X-ray and neutron powder diffraction, and 1H, 31P and 29Si nuclear magnetic resonance (NMR) spectroscopy. The composition of the Si-substituted HAp phases was determined by joint Rietveld refinements from powder X-ray and powder neutron diffraction data. Taking into account electroneutrality, a chemical formula for the Si-substituted HAp phases with indication of the incorporated silicate amount is proposed. Solid-state 29Si NMR confirms the presence of only Q0 species, in good agreement with the presence of substituted HAp and β-TCP phases only. Thanks to NMR spectroscopy, two types of protons in the Si-substituted HAp phase were identified, the new site corresponding to species engaged in hydrogen bonding with silicate anions. This allowed further refinement of the formulae for these phases with very good quantitative agreement for populations derived from the refinement and integration of NMR data.