A detailed electron and X-ray powder diffraction study has been made of the compositional and displacive flexibility of the rare earth zirconate pyrochlore (1−δ)ZrO 2 ·δLaO 1.5 =(La 3+ ) δ (Zr 4+ ) 1−δ O 2−δ/2 ,δ∼0.5. The extent of the pyrochlore “solid solution” in specimens rapidly quenched from 1500°C is shown to be considerably narrower than previously reported and certainly less than 2 mol.% in width. A weak but highly structured and quite reproducible diffuse intensity distribution in the form of {110} * polarized sheets of diffuse intensity perpendicular to each of the six 〈110〉 real space directions is observed in electron diffraction patterns and interpreted in terms ofβ-cristobalite-like 〈110〉 tetrahedral edge rotation of essentially rigid O(2)(La) 4 tetrahedra.