High temperatures in Solid Oxide Fuel Cells (SOFCs) place stringent requirements on the cell components materials which result in high material costs for interconnects and insulation and cause the degradation of the system. The development of Intermediate Temperature SOFCs (IT-SOFCs) will require electrolyte materials with higher ionic conductivity at moderate temperatures than the conventional yttria-stabilised zirconia (YSZ). Recently, lanthanum silicates with an apatite-like structure (La9,33Si6O26) have attracted considerable interest as potential materials for low cost electrolyte. Some of these materials show conductivities at 875 K comparable to, or better than, YSZ; their high level of oxide ion mobility was related to the presence of oxygen channels along the c axis which facilitate the diffusion of the anionic species (O2− for SOFCs applications). Another way to fulfil IT-SOFCs requirements is to decrease the electrolyte thickness to the micrometer range and, therefore, the ohmic drop at the electrolyte. Magnetron sputtering has already been used to synthesize thin film electrolytes for SOFCs owing to its versatility for depositing complex materials as well as its ability to control their composition and morphology. The manufacturing of thin film lanthanum silicate electrolytes by magnetron sputtering, with thicknesses in the micrometer range, can be achieved through the previous deposition of La-Si films and subsequent thermal oxidation. The present study focuses on the characterization of the morphological and structural changes upon oxidation of La-Si films deposited by magnetron sputtering.