A novel LISICON type gamma solid solution range has been found in the system Li4-2xCoxGeO4 for 0.15 less than or equal to x less than or equal to 0.8. The highest net conductivity, 8.4 x 10(-6) Scm(-1), was observed for x = 0.25 at 27 degrees C rising to 3.35 x 10(-3) Scm(-1) at 300 degrees C with an activation energy of 0.71 eV. A charge/discharge polarisation technique was used to investigate the transference number of these materials. Results corroborate those obtained by low frequency a.c. impedance measurements and show the occurrence of mixed Li+ ion and electronic conductivity. The relative amounts of each vary with composition; x = 0.25 has both the highest conductivity and the highest ionic transference number, 0.19.