Abstract X-Ray absorption fine structure (XAFS) studies were performed for cobalt(II) bromo complexes prepared for the ratio Br/Co from 0.0 to 4.0 and for their oxidized complexes with ozone in acetic acid solvent. The multivariate curve resolution method indicates the existence of two kinds of chemical species both before and after the oxidation. Before oxidation, the two species are cobalt(II) acetate and tetrahedral cobalt(II) complex coordinated by Br. The oxidized sample with ozone consists of cobalt(II) acetate and trinuclear cobalt(III) complex; however, cobalt(III) complexes including Br are not detected. The rate of the change from the cobalt(II) complex coordinated by Br to the trinuclear cobalt(III) complex is lower than that from cobalt(II) acetate complex. This indicates that the tetrahedral cobalt(II) complex coordinated by Br is more difficult to be oxidized than the octahedral cobalt(II) acetate complex.