The structure and composition of Ba0.5Sr0.5TiO3 thin films, sputter deposited on (001) MgO substrates, have been characterized by transmission electron microscopy. Deviations in film stoichiometry are seen to strongly correlate with the structural and dielectric properties of these films, with the films deposited at the lower sputtering pressures either Ti-deficient or capped with a titanium oxide layer similar to the rutile TiO2 phase. Preferential sputtering of cations is found to be an important factor governing film stoichiometry. The Ti-deficient films deposited at a lower sputtering pressure contain Ruddlesden-Popper faults that increase the average lattice constant of the film and result in compressive strain and low dielectric tunability. Published by Elsevier B.V.