Thermogravimetry, X-ray and electron diffraction and high resolution electron microscopy show that the reduction process from perovskite La0.5Ca0.5MnO3 to brownmilleriteLa0.5Ca0.5MnO2.5 is a reversible topotactic process. Two series of monophasic La0.5Ca0.5MnO3–δ oxygen-deficient perovskites were stabilized in the 0 ≤ δ ≤ 0.12 range, either from reduction of La0.5Ca0.5MnO3 or oxidation of La0.5Ca0.5MnO2.5. Soft X-ray absorption spectroscopy shows that Mn oxidation states differ as a function of the synthetic pathway. Temperature dependence of the magnetization as a function of anionic vacancies concentration at constant doping level are reported together with the evolution of high-resolution O K-edge and Mn L-edge spectra. Our results show dramatic changes in the magnetic behaviour due to the stabilization of different Mn oxidation states.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)