For formation of different oxygen nonstoichiometry in oxide samples NdSr2Mn2O7 ± δ applied a two-step synthesis method, which allows to achieve different oxygen nonstoichiometry of the samples. Oxygen nonstoichiometry is formed by two methods: quenching at certain temperatures, and annealing at certain values of oxygen partial pressure. The value of oxygen nonstoichiometry determined by the mass emitted from the sample to oxygen during the decomposition of samples NdSr2Mn2O7 ± δ to simple oxides. Determined oxygen nonstoichiometry in NdSr2Mn2O7 ± δ when recording external parameters such as pressure and temperature. The study of samples NdSr2Mn2O7 ± δ using thermogravimetry and x-rays revealed phase separation - two phases with the same cation composition, having different oxygen content. For example, sample NdSr2Mn2O7 ± δ , tempered 1000 °C and having an oxygen nonstoichiometry +0.09 proposed crystallographic criterion of manifestation of phase separation in phases Ruddlesden-Popper (R-P) - a sharp change in the bond lengths of the manganese-oxygen. Measurement of the electrical resistance of DC in the samples NdSr2Mn2O7 ± δ were performed in the temperature range 5 - 300 K and magnetic fields up to 7 T standard 4-probe method. Magnetoresistive effect up to 400 % was found in the studied oxides, which is nonlinearly dependent on their oxygen nonstoichiometry