Vacancy‐ordered Sr2Ti2O5(110) epitaxial filmsinsitudeposited on YBa2CU3O7−&dgr;(YBCO) (103) epitaxial films are studied for insulating layers in high‐Tcsuperconducting heterostructures. The ordered structure is interpreted by a 2×2×2 superstructure with a long‐range ordered arrangement of oxygen vacancies in the film and a 2×1 surface superstructure with an ordered arrangement of surface oxygen vacancies on it. The resistivity of the Sr2Ti2O5film sandwiched between Au and YBCO films is measured as a function of the thickness and temperature, and discussed with respect to electron scattering induced by a deficiency of oxygen in the film and at the film boundary. ©1995 American Institute of Physics.