The diffusion of Mg in pulsed laser deposited K(Ta0.65Nb0.35)O-3 thin films epitaxially grown on (1 0 0) MgO single crystal substrate were investigated by Auger electron spectroscopy (AES). A diffusion of Mg from the substrate into the whole thickness (400 nm) of the as-deposited K(Ta0.65Nb0.35)O-3 films was observed with an accumulation of Mg at the surface. Ex situ post-annealing (750 degrees C/2 h) has led to a homogeneous distribution of Mg in all the ferroelectric coating. This strong reaction between film and substrate promotes a doping effect, responsible for the reduction of K(Ta0.65Nb0.35)O-3 dielectric losses in comparison with films grown on other substrates. (C) 2011 Elsevier B. V. All rights reserved.