- The conduction mechanisms in yttrium aluminum and yttrium iron garnet solid solutions have been studied as a function of temperature, iron concentration and partial pressure of oxygen. At low concentrations of iron, ac conductivity and ionic transference measurements show the solid solution to be a mixed ionic-electronic conductor with an ionic mobility characterized by an activation energy of 2.6–2.8 eV and a p-type electronic conductivity with activation energy of 3.0–3.3 eV. High concentrations of iron cause a dramatic increase in the electrical conductivity connected with the formation of an Fe impurity band found to lie 1.9 eV below the conduction band. Transport through this band is via an activated hopping process with an activation energy of 0.7 eV for 6 fraction percent Fe. A defect model is presented which is consistent with our experimental observations including the conductivity maxima obtained at high PO2's for high Fe levels.