[en] Highlights: • Speciation of uranium in gadolinium zirconate. • Uranium stabilizes as U(IV) as well as U(VI). • U(VI) stabilizes as UO₆⁶⁻ ion in Gd₂Zr₂O₇. • XPS also confirms the presence of U in +6/+4 state but quantification is not possible. • Positron lifetime indicated formation of antisite defects. Based on photoluminescence spectroscopy it was inferred that uranium stabilizes as both U(IV) as well as U(VI) in Gd₂Zr₂O₇ which was also corroborated using X-ray photo electron spectroscopy (XPS). Absence of equidistant vibronic structure in emission spectrum of Gd_1.98U_0.02Zr₂O₇ confirmed that U(VI) stabilizes in the form of UO₆⁶⁻. Based on luminescence lifetime it was inferred that majority of UO₆⁶⁻ stabilizes at both Gd^3+/Zr⁴⁺ whereas U⁴⁺ stabilizes only at Zr⁴⁺ sites. The positron lifetime doesn’t change on uranium doping indicating the formation of antisite defect. Infact it is this antisite defect in Gd_1.98U_0.02Zr₂O₇ which favours the stabilization of its fluorite phase.