Abstract YBa 2Cu 3O 7− x films have been grown by reactive magnetron sputtering in an Ar + O 2 sputtering atmosphere. As-grown films were amorphous and insulating and require a post-deposition annealing to show superconducting transitions on cooling. The best zero-resistance temperature T 0 was measured to be 85 K with a transition width of 5 K. Josephson supercurrent tunneling was also observed in these films and was linked to the accumulation of the impurities at the grain boundaries. Cation stoichiometry was found to be dependent on the deposition conditions, especially the presputtering of the target prior to the deposition. Target surface analysis showed that a relatively thick layer has to be removed from the surface of the target before the true bulk composition can be achieved in the films.