This proposal requests 6 days beamtime on the Polaris diffractometer to study the nature of the local structure within two systems which have been reported to show a solid solution between pyrochlore and fluorite structure types. The first, Y2(Zr1-yTiy)2O7, shows a gradual transiton from a cation ordered to cation disordered arrangement as Zr content increases, forming an anion-deficient fluorite arrangement. In the second, Ce2Zr2O7+y, the replacement of Ce3+ with Ce4+ leads to a gradual increase in oxygen content, though the pyrochlore cation ordering remains even at the fluorite stoichiometry (Ce2Zr2O8). However, previous work on both these systems has focussed on the averaged structure and, to provide a detailed insight into the nature of the local co-ordinations around the various cation species, we shall perform reverse Monte Carlo modelling of the total scattering data