Various chemical oxidants were investigated for their capability to oxidize brownmillerite SrCoO 2:5 to perovskite SrCoO 3À . Among the oxidants tested, KMnO 4 /HCl was found to be the strongest to yield SrCoO 3À with % 0:06.The perovskite SrCoO 3À system is known for its wide range of oxygen nonstoichiometry. Hence, it serves as a kind of prototype for the recently highlighted functional oxides of mixed-valent (III/IV) cobalt. With the proper choice of process parameters it can be obtained as different types of metastable single phases with orthorhombic (0:5 < < 0:6), 1,2 tetragonal (0:1 < < 0:25) 2,3 or cubic ( % 0) 3-5 symmetry. Synthesis of the oxidized phases usually goes through the orthorhombic brownmillerite phase SrCoO 2:5 , 1 which has ordered oxygen vacancies such that the structure is composed of alternately stacked layers of corner-sharing CoO 4 tetrahedra and CoO 6 octahedra. For the oxidation of SrCoO 2:5 , electrochemical [3][4][5] and high-pressure 2,6 techniques have been employed. Possibility to utilize chemical ways for the control of oxygen content was brought up by Nemudry et al.3 who oxidized SrCoO 2:5 by immersing the sample into an aqueous solution of Br 2 and NaOH. Compared to the other methods chemical oxidation requires only a very simple experimental set up. In the present work, several other chemical oxidizing agents are utilized.The SrCoO 2:5 precursor was prepared from Co and SrCO 3 powders, dissolved in stoichiometric amounts in 3 M HNO 3 solution and then chelated by EDTA/NH 3 solution containing EDTA in 50% excess. The solution was dried, burned and then calcined at 900 C. The calcination product was ground and pressed into pellets for the final heat treatment in air at 1000 C for 40 h, followed by quenching to room temperature. The X-ray diffraction pattern for the product revealed a singlephase brownmillerite with an oxygen content of 2.50 per formula unit, as determined by iodometric titration.For the oxidation experiments, the SrCoO 2:5 pellets were crushed and ground. Four different oxidizing media were employed: (1) aqueous solution of 0.2 M KMnO 4 and HCl in various concentrations, (2) aqueous solution of Na 2 S 2 O 8 saturated at room temperature, (3) 1:1 dispersion of Br 2 and H 2 O, and (4) aqueous solution of 0.3 M Br 2 and 5 M NaOH. The experiments were carried out for 50-mg samples in 2-mL plastic vials except for experiment (4), which was carried out for a 200-mg sample in a 20-mL glass tube. Immersion time varied from 1 to 24 h. Temperature was controlled using a water or an ice bath and a thermometer. After the oxidation treatment, the sample powder was washed with deionized water plus ethanol [(1) and (2)] or CH 3 CN plus ethanol [(3) and (4)] and subsequently dried in air at room temperature.X-ray diffraction (XRD) patterns were collected for the samples with a Rigaku RINT2550 diffractometer equipped with a rotating Cu anode. Software JANA 2000 was used in the profile-fitting mode for the lattice-parameter refinements [in space groups, I4=mmm (SrCoO 3À ) and P...