The phase ‘NdMnO ₃ ’ has been synthesised and characterised by a combination of electron probe microanalysis, X-ray and neutron diffraction and H ₂ -reduction thermogravimetry. NdMnO ₃ forms, at ≈1400 °C, over the range Nd _1.00(1) Mn _0.95(1) O _2.92(1) to Nd _0.88(1) Mn _1.00(1) O _2.72(1) . Oxygen contents vary in air over the range 700 to 1400 °C and can be varied further, either by high pressure O ₂ treatment or by reduction in H ₂ . The structure of ‘NdMnO ₃ ’ is based on the GdFeO ₃ structure with a Jahn-Teller distortion associated with the high proportion of Mn ³⁺ ions present. However, it also shows a very varied defect structure; depending on composition and heat treatment, vacancies can form on any one or any two of the three sublattices, Nd, Mn and O and the overall Mn oxidation state can include 2+, 3+ and 4+ contributions. Data on compositional range and defect crystal structure are presented in the form of a novel phase diagram-defect structure map.