Gd 2(Ti 2- xZr x)O 7 samples with 0⩽ x⩽1.5 were single-phase and pyrochlore structured after sintering at 1600°C in air. The Gd 2Zr 2O 7 ( x=2) end member predominantly displayed an anion deficient-fluorite structure. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd 2(Ti 2- xZr x)O 7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0⩽ x⩽1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio rA/ rB⩽1.52( x⩾1.5) were transformed into a radiation resistant defect-fluorite structure after irradiation at room temperature with 2 MeV Au2+ to a fluence of 5 ions/ nm2. As the ionic radii ratio of the pyrochlore increased beyond rA/ rB>1.52, the defect-fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.