A solid solution of the lithium superionic conductor Li_10+δGe_1+δP_2−δS₁₂ (0 ≤ δ ≤ 0.35) was synthesized and its structure and ionic conductivity were examined. The highest ionic conductivity value of 1.42 × 10⁻² S cm⁻¹ was obtained at 300 K with a sintered pellet of the sample having the highest solid solution lithium content of δ = 0.35. The Arrhenius conductivity curves obtained for this material exhibited a gradual change in slope over the temperature range of 193–373 K and the activation energy for ionic conduction decreased from 26 kJ mol⁻¹ below 373 K to 7 kJ mol⁻¹ above 573 K, which is typical of highly ionic conducting solids. The crystal structures of the solid solutions were determined using neutron diffraction, and conduction pathways were visualized through analysis by applying the maximum entropy method. The lithium distribution was found to disperse significantly throughout a one-dimensional conduction pathway as the temperature was increased from 4.8 K to 750 K. In addition, two-dimensional distribution of lithium along the ab plane became apparent at high temperatures, suggesting that the conduction mechanism changes from one-dimensional to three-dimensional with increasing temperature.