Phases with the cubic pyrochlore structure and the composition Tl2Nb2O6+x were prepared under various oxygen pressures. Rietveld refinements of neutron powder diffraction data showed 10.674 Angstrom for x = 0.11, a = 10.637 Angstrom for x = 0,44, and a = 10.627 Angstrom for x = 0.64. Reflections forbidden by spherical atoms in ideal positions are observed at x = 0.11, and these can be attributed to an anisotropic thermal displacement parameter for Tl. Despite the Tl(I)/Tl(III) mixed valency, these phases show a high electrical resistivity. (C) 2000 Academic Press.