Epitaxial La0.35Nd0.35Sr0.3MnO3 (LNSMO) films and SrTiO3 (STO)/LNSMO heterostructures have been grown on LaAlO3 substrates by pulsed laser deposition. The effect of oxygen content on structure and properties of both the LNSMO and STO/LNSMO films was investigated through x-ray diffraction, atomic force microscopy, and resistivity-temperature measurements. It is found that the out-of-plane lattice constant and the metallic-semiconducting transition temperature of the LNSMO films are greatly influenced by the oxygen pressure during deposition, but, insensitive to the in situ annealing oxygen pressure ranging from 2×10-6 to 10 Torr after the deposition. For the STO/LNSMO heterostructures, oxygen out-diffusion from the LNSMO layer is evidenced when the top STO is deposited at an oxygen pressure of less than 5×10-4 Torr and temperatures higher than 500 °C. Our results strongly suggest that at the surface of as-grown LNSMO films a native passivation layer for oxygen diffusion may exist, and this layer could be damaged after depositing the STO film at reduced oxygen pressures. © 2000 American Vacuum Society.