Lanthanum-substituted strontium-oxybritholites, Sr$_{10-x}$La$_x$(PO₄)$_{6-x}$(SiO₄)$_{x}$O with $x = 0$, 2 and 4, prepared by solid state reaction were investigated by chemical analysis, powder X-ray diffraction, Raman and ²⁹Si MAS NMR spectroscopies. The refinements of powder XRD patterns of the substituted compounds by the Rietveld method showed that the lanthanum occupied the two metal sites, i.e. (4f) and (6h) sites into the apatite structure, with a clear preference for the (6h) sites. A progressive shift of the free oxygen O(4) towards the centre of the triangles formed by the metal-atoms in the (6h) positions was observed when the lanthanum content increased. It led to the formation of a Sr/La(2)-O(4) strong bond, which might have increased the stability of these compounds. The bands of Raman spectra were assigned to the vibration modes of PO₄ and SiO₄ groups. The comparison of the results of ²⁹Si MAS NMR analysis with those obtained with the ³¹P previously reported, suggested that both species occupied the same crystallographic sites.