A brownmillerite SrFeO2.5 thin film grown on a SrTiO3(001) substrate contained an a-axis-oriented single-crystalline domain near the substrate and multiple domains with three orthogonal orientations. When the film was reduced with CaH2. it changed topotactically to a homogeneous c-axis-oriented infinite-layer structure, SrFeO2. A brownmillerite SrFeO2.5 thin film grown on a SrTiO3(111) substrate and treated with CaH2 at 250 degrees C also changed to (111)-oriented SrFeO2 with an infinite-layer structure. During reduction at low temperatures. the framework of the perovskite-structure stays intact, some oxygen atoms are removed from it. and others are rearranged.