The thermal reactivity of the perovskite-related phase YBaFeCuO5 with a partial substitution of Fe or Cu by Co was investigated by applying different reactive gas atmospheres. The degradation of the structure under reducing conditions (5%H-2/N-2) depends on the composition of the samples including mixed valences attributed to Co(II)/Co(III). From powder X-ray diffractiometry of the decomposed product of YBaCoCu0.6Fe0.4O5+delta, a selective reduction of Co, Cu and the formation of a new ternary phase were detected. The reduction products easily reformed a perovskite phase under O-2, as shown by TG-DTA and XRD. Moreover, thermogravimetric studies proved the reversible incorporation and release of oxygen into the oxygen-deficient layered perovskite structure. An application for catalytical purposes is suggested.