We report that the sign of the polarization of an epitaxial ferroelectric film can be determined from the interference between the x-ray scattering from the film and the substrate. X-ray scattering measurements of a 10 nm epitaxialPbTiO3film grown by metal-organic chemical vapor deposition on aSrTiO3substrate are presented. The scattering profile near the001peaks of the film and substrate shows clear evidence of the interference effects. Analysis indicates that this film is a single domain of specific polarity. ©1997 American Institute of Physics.