Oxide ion diffusivities were evaluated in single crystals of Mg-Al-Fe-O spinel solid solutions. The single crystals were grown by a traveling solvent floating zone technique. The diffusion coefficients of oxide ions were obtained by means of a solid-gas exchange method, using ¹⁸O isotope as tracer. The secondary mass spectrometer was used to measured the oxygen diffusion profiles in solids. Some profiles was not able to be interpreted by a simple error function. These anomalies are considered to be due to oxidizing or reducing process in the single crystals during the oxygen diffusion annealing. Iron ions preferentially diffused to surface during oxygen diffusion annealing in Mg_0.93Al_1.24Fe_0.83O_4+δ-crystals, while Mg_0.91Al_0.72Fe_1.37O_4+δ-crystals have not these behaviors. The resultant diffusion coefficient of Mg_0.91Al_0.72Fe_1.37O_4+δ-crystals were divided two temperature dependence as follows;
D=2.0×10^-1exp(-372(kJ/mol)/RT)cm²⋅s^-1>1530K
D=1.0×10^-7exp(-181(kJ/mol)/RT)cm²⋅s^-1<1530K
This tendency agreed with the polycrystalline data. In these samples it is considered that the oxide ions diffuses through oxygen vacancies.