An oxide ion conductor, La2Li2x(CO3)1−xO2+x (x=0.15), was synthesized using a solid-state reaction, and its structure was subsequently determined by Rietveld refinement analyses of synchrotron X-ray and high-resolution powder neutron diffraction data. These analyses determined that carbonate groups are situated between (La2O22+)n layers in an ordered arrangement and that both oxygen vacancies and interstitial oxygen ions exist within the carbonate layers, resulting in a composition of La2Li0.3(CO3)0.85O2.3. This material is structurally stable up to 500 °C and exhibits a conductivity of 5.5×10−5 S cm−1 at 450 °C. Oxide vacancies associated with carbonate anions appear to be responsible for the ionic conduction observed in this material.