The mixed cation system, Li10(Ge1−x M x )P2S12 (M = Si, Sn), was synthesized and the ionic conductivities of the resulting solid solutions were determined. The Si–Ge and Ge–Sn systems provided single phase solid solutions for the composition 0 ≤ x < 1.0 and 0 ≤ x ≤ 1.0 in Li10(Ge1−x Si x )P2S12 and Li10(Ge1−x Sn x )P2S12, respectively. The lattice size gradually increased from Si to Sn through the Ge systems, reflecting the ionic size of these elements. On the other hand, conductivity did not follow the increase in lattice volume. Conductivity increased in the Si to Ge system, with the maximum conductivity value of 8.6 × 10−3 S cm−1 provided by the compressed powder with the composition Li10Ge0.95Si0.05P2S12, which is close to the original Li10GeP2S12 (LGPS) composition. The conductivity decreased with increasing Sn content, indicating that lattice volume is not the only parameter that affects ionic conduction in this structure. The relationship between conductivity and lattice volume is discussed.