Sr-deficient and stoichiometric epitaxial (001) SrTiO3 films, deposited on (110)rhombohedral LaAlO3 substrates by rf magnetron sputtering, have been characterized using high-resolution transmission electron microscopy. A subsequent heat treatment in oxygen had a positive influence on the dielectric properties. Sr-deficiency had a large negative impact on the microwave dielectric constant of the films. These changes were correlated to changes in lattice parameters. In all samples, at the film/substrate interface, were misfit dislocations present. The residual elastic strain compressed the SrTiO3 unit cell in the substrate surface plane and expanded it an equal amount in the  direction. X-ray diffraction revealed that the tetragonal distortion, due to the mismatch strain, was concentrated to a narrow region closest to the film/substrate interface. © 1998 American Institute of Physics.