We present a Raman spectroscopy study of the phase transition occurring in a BaSrTi O thin film deposited by ablation laser on a MgO substrate. This Raman investigation is compared to x-ray diffraction studies of the temperature dependence of the structure and of the dielectric properties at microwave frequencies. These different probes evidence a diffuse phase transition in the range of 243-283 K from the low temperature ferroelectric-tetragonal phase to the high temperature paraelectric cubic phase. Stabilization of the tetragonal phase in the thin film down to 83 K was observed instead of the expected transition to an orthorhombic phase below 190 K. This stabilization may be attributed to the tensile stress induced by the substrate. A careful analysis of the frequency dependence of the dielectric response suggested that the observed tunability and dielectric loss were determined by extrinsic effects such as charged defects and/or depletion layers. © 2008 American Institute of Physics.