Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. Substituted silicated rare earth oxyapatites with formula La9.33+z/3−xMex□0.67−z/3(SiO4)6O2+z/2−x/2 (z < x < z + 4) have been prepared by solid-state reaction at high temperature. Two series have been synthesized: a first one is oxygen stoichiometric with formula La9.33−2x/3Mex□0.67−x/3(SiO4)6O2, and a second one is anion deficient with formula La9.17−2x/3Mex□0.83−x/3(SiO4)6O1.75□0.25. In both cases, cationic vacancies are similarly controlled and vary from 0.67 to 0 per unit cell: the aim is to study the influence of cationic vacancies on the ionic conductivity with two distinct oxygen stoichiometries. Cell parameters of the high-purity oxyapatites have been refined in order to check the strontium incorporation. Discontinuous evolution of the a parameter underlined the strong electrostatic interactions between the defects of the most highly substituted samples. Electrical properties of the samples have also been studied by the complex impedance method between 280 and 620 °C. The evolution of conductivity and activation energy with the cationic vacancies content gives information on the conductivity mechanism, highlighting the importance of the global stoichiometry of the material.