Oxides are believed to assume the A(2)B(2)O(7) pyrochlore structure type for a specific range of ratios of the cation radii, R-A/R-B. Substitution of a larger B' ion in solid solution for B can progressively drive the system to complete disorder as in Y-2(Zr-y Ti1-y)(2)O-7, producing an oxygen-ion conductivity, sigma greater than 10(-2) S/cm at 1000 degrees C. comparable to the values of 10(-1) S/cm found for M3+-stabilized cubic zirconias. Rietveld analyses of neutron and X-ray powder diffraction data have been employed to obtain structural data for the related systems Y-2(SnyTi1-y)(2)O-7, Y-2(ZrySn1-y)(2)O-7, Gd-2(SnyTi1-y)(2)O-7 and (SczYb1-z)(2)Ti2O7 to test whether the state of disorder and attendant ionic conductivity are indeed determined by R-A/(R-B,R-B) This was not the case for the Sn-Ti solid solutions: they retained an ordered pyrochlore structure for all values of y, The slight variation of ionic conductivity (less than one order of magnitude with a maximum in sigma at intermediate y) was successfully explained by the structural data. The behavior of Y-2(ZrySn1-y)(2)O-7 solid solutions was very similar to that of the Zr-Ti phases. Neutron powder diffraction profiles were recorded as fully-ordered Y2Sn2O7 and highly-disordered Y-2(Zr0.6Ti0.4)(2)O-7 were heated in situ at temperatures in the range 20-1500 degrees C. The structure of Y2Sn2O7 steadfastly remained fully-ordered over this temperature range. The principal change in the structure was increase in the positional coordinate, x, for O(1), corresponding to increased distortion of the oxygen-ion array as temperature was increased, a consequence of greater thermal expansion of the A(3-)-O bond relative to change in the B4+-O separation. The highly-disordered cation arrangements in Y-2(Zr0.6Ti0.4)(2)O-7 remain unchanged up to 1250 degrees C when the oxygen array began to undergo further disorder. The same anion site occupancies were observed during heating and cooling cycles suggesting that their distribution does represent an equilibrium state.