La-Si thin films were deposited oil stainless steel substrates by magnetron Sputtering from Pure La and Si targets. The Si/(Si+La) atomic ratio in the films was varied from 43.2 to 59.3% by adjusting the discharge current on the Li target. The films had a homogeneous chemical composition down to the substrate and sharp interfaces. Annealing the films in air at 1173 K promotes the formation of apatite-structure La(9.33)Si(6)O(26) and the diffusion of different species from the film to the substrate and vice-versa, resulting in broadening the interfaces. X-Ray diffraction showed that all the as-deposited films had an amorphous structure. The formation of the LaSi(2) phase at intermediate temperatures was observed for the films deposited with higher Si contents while the films deposited with lower Si contents remained amorphous LIP to the Start Of the apatite structure crystallization process. The lanthanum silicate apatite-like phase (La(9.33)Si(6)O(26)) was obtained only after annealing at 1173 K, excepted for the film with the lower Si content which is already partially crystallized after annealing at 1073 K. Quite pure La(9.33)Si(6)O(26) was obtained only after annealing the film with the highest Si content (Si/(Si+La)=59.3%) although the theoretical Si/(Si+La) atomic ratio for apatite structure lanthanum silicate is 39%. For the other films, La(2)O(3) was always detected when the lanthanum silicate phase was formed. Both phenomena clearly resulted from the strong diffusion of silicon excess towards the stainless steel substrate. (C) 2008 Elsevier B.V. All rights reserved.