The growth of (001) oriented Sr (x) Ba1-x Nb-2 O-6 (SBN:x) thin films on MgO single crystals substrates has focused attention due to its potential application to integrated signal waveguiding and electro-optic processing. This paper addresses the possible insertion of a platinum bottom electrode to implement the electro-optic effect, and examines the influence of Pt orientation on the subsequent growth of SBN thin films deposited by R.F. magnetron sputtering. We have first investigated the optimal sputtering conditions of a (001) Pt oriented growth. Experimental evidence is provided for an orientation switching temperature sensitive to R.F. power but insensitive to deposition rate which suggests that kinetic phenomena are not involved in the orientation development of Pt films. Seeding the MgO substrate with Fe, Cu or Cr inhibits the competitive (111) Pt growth without modifying the optimal temperature for (001) growth. Annealing at a temperature sufficient to crystallise the top SBN film causes a drastic evolution of the initial (111) and (001) Pt volume fractions, making particularly critical the deposition conditions for a final dominant (001) Pt texture on unseeded substrates. The (001) oriented growth of SBN is shown to occur exclusively on (001) Pt crystallites stable against subsequent annealing steps. As a final result, (001) SBN films have been obtained epitaxial on both Pt coated and not coated (001) MgO substrates. They display two in-plane orientations mirror symmetric (+/-alpha) to the axis of (001) MgO, with alpha shifted from 31 degrees to 18 degrees by (001) Pt coating.