A new range of stable γ solid solutions has been found in the system , of formula . For compositions, , a range of solid solutions based on also forms. At low , the solid solutions are predominantly electronic conductors but with increasing , the level of electronic conductivity decreases giving rise to materials that are either mixed conductors or ionic conductors, depending on composition. Thus, composition has good Li+ conductivity, at 25°C where is primarily an electronic conductor, at 25°C. These materials provide the first example of mixed conduction in the LISICON‐related γ solid solution family. A new qualitative method for investigating transport numbers in mixed conductors is proposed: dc polarization measurements are made using pelleted samples with Au electrodes and the amount of charge stored as a consequence of electrochemical decomposition of the pellet is estimated; results obtained from such charge/discharge experiments corroborate those obtained by low frequency ac impedance studies. Samples which are primarily electronic conductors give rise to significant Schottky barrier impedances at the sample‐electrode interface. These effects are also investigated by ac impedance by varying the nature of the electrode materials: In/Ga or Au.