KTa1−xNbxO3 (KTN) thin films were grown by pulsed laser deposition on sapphire and MgO substrates. Theirstructural and high frequency dielectric characteristics evidenced the strong influence of the substrate andsuggested possible KTN/MgO interdiffusion that could be responsible for the lower dielectric losses obtained onthis substrate. Both undoped and 6% MgO-doped KTN thin films were then grown on sapphire. Dielectricmeasurements performed at 12.5 GHz by a resonant cavity perturbationmethod evidenced reduction of losses byMgO-doping. Loss tangent (tan δ) was reduced by a factor of 3 in comparison with undoped films grown onsapphire.© 2009 Elsevier B.V. All rights reserved.