Lanthanum silicate apatite materials, which are good ionic conductors, were prepared and characterized in two series, LSO: La9.33+x(SiO4)(6)O2+3x/2 (x=0, 0.33 and 0.67) and LSSO: La10-xSrx(SiO4)(6)O3-0.5x (x=1.00 and 2.00). Enthalpies of formation from their binary oxides at 298 K were determined by high temperature oxide melt solution calorimety. The energetics of these materials is discussed in terms of the effects of cation vacancies and interstitial oxygens. The formation of LSO apatites becomes more exothermic as the number of cation vacancies decreases (oxygen excess increases). Cation vacancy content is the dominant factor in determining the energetics of La9.33+x(SiO4)(6)O2+3x/2. The endothermic enthalpy of defect formation is 133.8 +/- 17.5 kJ per mole of interstitial oxide ions, and 272.2 +/- 21.6 kJ per mole of cation vacancies. Thus. La8Sr2(SiO4)(6)O-2, having neither vacancies nor interstitial oxide ions, is the most stable compound in these two series. (C) 2013 Elsevier B.V. All rights reserved.