A new range of stable gamma solid solutions has been found in the system Li4SiO4-Li3CrO4, of formula Li3+xSixCr1-xO4:0.3 < x < 0.65. For compositions, 0 < x less than or equal to 0.4, a range of solid solutions based on beta-Li3CrO4 also forms. At low x, less than or equal to similar to 0.5, the solid solutions are predominantly electronic conductors but with increasing x the level of electronic conductivity decreases giving rise to materials that are either mixed conductors or ionic conductors, depending on composition. Thus, composition x = 0.7 has good Li+ conductivity, 1.7 x 10(-6) Ohm(-1) cm(-1) at 25 degrees C where x = 0.5 is primarily an electronic conductor, 2.3 X 10(-5) Ohm(-1) cm(-1) at 25 degrees C. These materials provide the first example of mixed conduction in the LISICON-related gamma solid solution family. A new qualitative method for investigating transport numbers in mixed Li+/e(-) conductors is proposed : de 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.