A range of high Li+ ion conductivity solid solutions with the γ-LISICON structure forms by the substitution mechanism 3Li⇄Ga. These extend from compositions close to, but not including, Li4GeO4 up to Li2.5Ga0.5GeO4. The phase diagram for the join Li4GeO4-Li2.5Ga0.5GeO4 is presented together with lattice parameter data on the γ solid solution, a new phase β-Li2.5Ga0.5GeO4 and an ordered γ phase, c-Li2.8Ga0.4GeO4. Conductivity Arrhenius plots are markedly non-linear. A linear region is observed below ∼260 °C in which the conductivity data depend on sample pretreatment. A high temperature curved region is seen in which the data are reversible on heating/cooling cycles. Conductivities reach a maximum at the approximate composition Li3.7Ga0.1GeO4.