Electrolytes based on pyrochlores such as Ln2M2O7 (M = Ti, Zr, Hf) have attracted attention due to their relative stability combined with relatively high ionic conductivity at intermediate temperatures, with a view to their use in solid oxide fuel cells. Subvalent substitution of the larger A site cation introduces oxide ion vacancies into the system with a consequent significant increase in oxide ion conductivity. Additionally dielectric permittivity measurements suggest subtle phase transitions at elevated temperatures are enhanced in the substituted systems. A variable temperature total neutron scattering study of oxide vacancy distribution in the system (Nd1-xCax)2Zr2O7-x is proposed, through a combined Reverse Monte Carlo analysis of neutron and X-ray scattering data. Details of the cation distribution will be examined through parallel measurements isotopically enriched 43Ca.