The solubility of Ce in the La1-xCexSrGa3O7+delta and La1.54-xCexSr0.46Ga3O7.27+delta melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce3+ into Ce4+. Under CO reducing atmosphere, the La in LaSrGa3O7 may be completely substituted by Ce to form the La1-xCexSrGa3O7+delta solid solution, which is stable in air to similar to 600 degrees C when x = 0.6. On the other side, the La1.54-xCexSr0.46Ga3O7.27+delta compositions displayed much lower Ce solubility (x = 0.1), irrespective of the synthesis atmosphere. In the as-made La1-xCexSrGa3O7+delta, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce3+ cations. At 600 degrees C, CeSrGa3O7+delta showed a conductivity of similar to 10(-4) S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa3O7. The oxidation of Ce3+ into Ce4+ in CeSrGa3O7+delta slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa3O7+delta. The Ce doping in air also reduced the interstitial oxide ion conductivity of La1.54Sr0.46Ga3O7.27. Neutron powder diffraction study on CeSrGa3O7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO4 polyhedral environment, leading to distorted GaO5 trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa3O7+d, in contrast with those in La1+xSr1-xGa3O7+0.5x, may be correlated with the cationic size contraction from the oxidation of Ce3+ to Ce4+. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure.