Higher-order Ruddlesden-Popper phases, however, tend to be understoichiometric and, therefore, oxygen vacancies are dominant defects. In comparison to lower-order Ruddlesden-Popper phases, there are very few neutron diffraction studies of lanthanide-based higher-order Ruddlesden-Popper phases such as La0.3Sr2.7CoFeO7- (n = 2), and LaSr3Co1.5Fe1.5O10- (n = 3) available in the literature . This report will, therefore, detail our recent neutron diffraction studies and complimentary DFT calculations of three n = 3 Ruddlesden-Popper compositions -La3PrNi3O10- (L3P1N3), La2Pr2Ni3O10- (L2P2N3) and LaPr3Ni3O10- (L1P3N3)to provide a comprehensive study of oxygen defect structure and transport properties in these materials. The higher-order phases tend to be oxygen understoichiometric in nature and therefore oxygen vacancies are predominant defects in these materials. The three phases under discussion too were previously reported by our group to be oxygen deficient in nature .