AbstractTo create different oxygen nonstoichiometry in oxide samples of NdSr2Mn2O7 + δ, we have used a two-step method of synthesis, which makes it possible 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 is determined by the mass of oxygen emitted from the sample during the decomposition of samples of NdSr2Mn2O7 + δ to simple oxides. We have determined oxygen nonstoichiometry in NdSr2Mn2O7 + δ when recording external parameters such as pressure and temperature. The study of samples of NdSr2Mn2O7 + δ using thermogravimetry and X-rays revealed phase separation—two phases with the same cation composition, having different oxygen content. By example of a sample of NdSr2Mn2O7 + δ tempered from 1000°C and having oxygen nonstoichiometry of +0.09, we have proposed a crystallographic criterion of manifestation of phase separation in Ruddlesden–Popper (R–P) phases—a sharp change in the manganese-oxygen bond lengths. Measurements of the electrical dc resistance in the samples of NdSr2Mn2O7 have been performed in the temperature range of 5–300 K and magnetic fields up to 7 T using the standard 4-probe method. A magnetoresistive effect up to 400% was found in the studied oxides, which is nonlinearly dependent on their oxygen nonstoichiometry.