Uranium plutonium Mixed oxide Containing 30% of plutonium is a candidate fuel for several fast neutron and accelerator driven reactor systems. In this work, a detailed Structural investigation on,sol gel-synthesized stoichiometric U0.7Pu0.3O2.00 an substoichiometric U0.7Pu0.3O2-x, using X-ray diffraction (XRD), oxygen 17 magic angle spinning nuclear magnetic resonance (O-17 MAS NMR) and X-ray absorption spectroscopy is described. As observed by XRD, the stoichiometric U0.7Pu0.3O2.00 is rrronophasic with a lattice parameter in good, agreement with Vegard's law, while the substoichiometric U0.7Pu0.3O2-x material is biphasic. Solid solution ideality in terms of a random distribution of metal atoms is proven for U0.7Pu0.3O2.00 with O-17 MAS NMR X-ray absorption near-edge structure ()CANES) spectroscopy shows the presence of plutonium(III) in U0.7Pu0.3O2-x, Extended X-ray, absorption fine structure (EXAFS) spectroscopy indicates a similar local structure around both cations, and comparison with XRD indicates a close similarity between uranium and plutonium local structures and the long-range ordering.