Epitaxial thin films of YBa2Cu3O7-delta (Y-BCO) have successfully been grown by reactive RF-magnetron sputtering technique at a low pressure (0.95 mTorr) with T-C (R = 0) = 85.3 K and J(C)(mag) C (4.2 K) approximate to 2 X 10(7) Angstrom/cm(2). The incorporation of oxygen in the as-grown films has been controlled by using different ambient-oxygen, air and argon-during in situ cooling. The superconducting behavior of the resulting films was studied using resistance-temperature and low field a.c.-susceptibility measurements and correlated with their structure. All the films exhibited metallic conduction in the normal state. The oxygen- and air-cooled films were superconducting, and exhibited usual orthorhombic structure, but the argon-cooled films were non-superconducting and possessed tetragonal structure. This implied that the structure of the film during deposition is tetragonal, and transforms to either of the oxygen rich orthorhombic-I or -II phases depending upon the oxygen/air ambient. The W values of 0.14, 0.32 and 0.70 and higher V-parameters of 1.1785, 1.180 and 1.183 nm have been obtained for oxygen-, air- and argon-cooled films, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.