Oriented, single phase thin films (∼5000 Å thick) of Ba0.5Sr0.5TiO3 (BST) have been deposited onto (100) MgO and (100) LaAlO3 (LAO) substrates using pulsed laser deposition. The capacitance and dielectric Q (1/tan δ) of as-deposited and annealed films have been measured from 1 to 20 GHz as a function of electric field (0–80 kV/cm) at room temperature using interdigitated Ag electrodes deposited on top of the film. For films deposited onto MgO, it is observed that, after a postdeposition anneal (1000–1200 °C), the dielectric constant decreases and the dielectric Q increases. For films deposited onto LAO, a postdeposition anneal (⩽ 1000 °C) resulted in a significant increase in the dielectric constant and a decrease in Q. The observed dielectric properties of the BST films are attributed to the changes in film stress, which affects the extent of ionic polarization. © 1999 American Institute of Physics.