Two phase samples containing∼ 70 vol.% Ga 2 O 3 and∼ 30 vol.% yttria-stabilized zirconia (YSZ) were fabricated by sintering powder compacts in air at 1500 C for 1 h and at 1600 C for 1 h and 3 h. Single phase samples of Ga 2 O 3 were fabricated by sintering at 1500 C for 1 h. Sintered samples were packed in Na-β''-alumina powder of composition∼ 8.85 wt% Na 2 O, 0.75 wt% Li 2 O and 90.4 wt% Al 2 O 3, and were heat treated at 1250 C for up to 20 h. The two phase Ga 2 O 3+ YSZ samples converted into a sodium zirconium gallate of composition Na 0.7 Ga 4.7 Zr 0.3 O 8 and balance yttria-stabilized zirconia. The reaction of single phase Ga 2 O 3 was limited to a thin surface layer in which Na-β''-gallate formed. Rapid conversion of Ga 2 O 3+ YSZ samples into Na 0.7 Ga 4.7 Zr 0.3 O 8+ YSZ occurred by coupled diffusion of O 2− ions through YSZ and of Na+ ions through the formed Na 0.7 Ga 4.7 Zr 0.3 O 8, similar to the conversion of α-Al 2 O 3+ YSZ into Na-β''-alumina+ YSZ. The phase Na 0.7 Ga 4.7 Zr 0.3 O 8 is isostructural with Na 0.7 Ga 4.7 Ti 0.3 O 8 which is a one dimensional sodium ion conductor. The conductivity of two phase Na 0.7 Ga 4.7 Zr 0.3 O 8+ YSZ composite was measured by Electrochemical Impedance Spectroscopy.