Silicate substituted hydroxyapatite bioceramics have been shown to enhance bone repair in vivo compared to hydroxyapatite (HA), although the amount of silicate ions that can be substituted alone into the hydroxyapatite structure is limited to approximately 5.2 wt%, or 1.6 wt% Si. This study describes the substitution of greater levels of silicate ions via co-substitution of silicate ions with trivalent yttrium ions, without resulting in the formation of any secondary phases. This substitution mechanism involves a coupled substitution of yttrium and silicate ions for calcium and phosphate ions, respectively, and enables a level of silicate substitution up to approximately 9 wt%. Two different substitution mechanisms result in subtle differences in the crystal structure. When the mechanism xY3+ + xSiO4 4- was used, a small decrease in the a-axis, but no change in the c-axis, of the unit cell compared to HA was observed. In contrast, when the mechanism x/2Y3+ + xSiO4 4- was used, a significant increase in the c-axis of the unit cell was observed, compared to HA. XRF analysis and FTIR spectroscopy supported the proposed substitution mechanisms. These novel substitution mechanisms not only enable greater levels of silicate-substitution in HA to be prepared, but also allow the production of compositions with the same level of silicate substitution, and with subtle differences in chemical structure.