Ba1-xSrxTiO3(0.4less than or equal toxless than or equal to0.8) is attractive for microwave device applications due to its non-linear dielectric response in d.c. bias fields. The maximum tunability {(C-0-C-nu)/C-0} with low dielectric losses is normally observed in the paraelectric phase close to the ferroelectric transition temperature. For the microwave transmission, the insertion loss in the device has to be minimized. In an effort to bring down the insertion loss, we have synthesized and characterized the heterostructured Ba0.5Sr0.5TiO3 {BST50} thin films with intermediate low loss MgO layers using sol-gel technique. Three different BST50:MgO hetrostructured films (type I, II and III) with different BST and MgO film sequences and thicknesses (and hence different BST:MgO volume ratios) were prepared and structural, microstructural, and dielectric properties of these films were investigated. The microstructure of the film changes appreciably with the insertion of MgO layers of different thicknesses in the BST/MgO heterostructures. The dielectric losses, in general, were found to reduce and hence the figure of merit at low frequency (K factor=tunability/loss tangent) of the heterostrcutured films was found to improve with the MgO insertion in BST films. The studies revealed an optimum thickness of MgO intermediate layers in the BST films for obtaining the K factor of similar to55 with electric field of 25.3 kV/cm, measured at 1 MHz. (C) 2003 Elsevier B.V. All rights reserved.