New solid electrolytes are crucial for the development of all-solid-state lithium batteries with advantages in safety and energy densities over current liquid electrolyte systems. While some of the best solid-state Li+-ion conductors are based on sulfides, their air sensitivity makes them less commercially attractive, and attention is refocusing on air-stable oxide-based systems. Among these, the LISICON-structured systems, such as Li2+2xZn1-xGeO4 and Li3+xV1-xGexO4, have been relatively well studied. However, other systems such as the Li4GeO4-Li2MoO4 system, which also show LISICON-type structures, have been relatively little explored. In this work, the Li4-2xGe1-xMoxO4 solid solution is investigated systematically, including the solid solution limit, structural stability, local structure, and the corresponding electrical behavior. It is found that a γ-LISICON structured solution is formed in the range of 0.1 ≤ x < 0.4, differing in structure from the two end members, Li4GeO4 and Li2MoO4. With increasing Mo content, the β-phase becomes increasingly more stable than the γ-phase, and at x = 0.5, a pure β-phase (β-Li3Ge0.5Mo0.5O4) is readily isolated. The structure of this previously unknown compound is presented, along with details of the defect structure of Li3.6Ge0.8Mo0.2O4 (x = 0.2) based on neutron diffraction data. Two basic types of defects are identified in Li3.6Ge0.8Mo0.2O4 involving interstitial Li+-ions in octahedral sites, with evidence for these coming together to form larger defect clusters. The x = 0.2 composition shows the highest conductivity of the series, with values of 1.11 × 10-7 S cm-1 at room temperature rising to 5.02 × 10-3 S cm-1 at 250 °C.