Using magnetron sputtering and annealing techniques, we have prepared SrCoOx films on LaAlO3 and SrTiO3 substrates. Distinctly different structures of the films have been found on the two substrates. It is suggested that positive lattice mismatch between film and substrate promotes SrCoO2.5 films with an orthorhombic structure grown on SrTiO3 substrate, whereas negative lattice mismatch from LaAlO3 substrate is in favor of increasing the valence state of Co and thus the growth of oxygen-rich SrCoOx with a tetragonal structure. In addition to the structural characterization, magnetic and electrical measurements confirm that the oxygen content x is between 2.75 and 2.88 for the latter. Reversibility of the topotactic phase transformation between SrCoO2.5 and the oxygen-rich SrCoOx films has also been studied by changing the oxygen pressure during annealing process. Even in the presence of a negative lattice mismatch, the results reveal that the tetragonal SrCoOx films on LaAlO3 substrate retain high oxygen mobility identified before in cubic SrCoOx films. (C) 2015 Elsevier B.V. All rights reserved.