Substituted perovskite-type mixed oxides of the general formula La1-xSrx(Fe3+/Fe4+)O3+/-6 were prepared for x = 0.00, 0.15, 0.30, 0.40, 0.60, 0.70, 0.80, and 0.90 by the ceramic method and heating at 1000degreesC. The prepared solids were examined by X-ray diffractogram (XRD) for their crystal structure and by Mossbauer spectroscopy at RT for the kind and the exact amount of iron containing crystal phases. The gradual substitution of La3+ by Sr2+ results in a quantitative development of the crystal phase SrFe4+O3 and the corresponding diminution of the rest phases containing iron, i.e. LaFeO3 and Fe2O3 (traces). Detailed thermogravimetric O-2/temperature-programmed desorption (TPD) experiments showed that the solids release oxygen when heated in a He atmosphere which is strictly proportional to the amount of Fe4+ in the samples. Quantitative calculations showed that exactly one atom of oxygen is released per unit cell of Sr-Fe-O perovskite, which contains four Sr(Fe4+/Fe3+)O3+/-delta units. The phenomenon is reversible and the solids re-adsorb fast and quantitatively the lost lattice oxygen when gaseous oxygen is admitted back to the system.