Systematic explorations of new phases in the Ga^III/In^III–Mo^VI–Se^IV/Te^IV–O systems by hydrothermal syntheses or solid-state reactions at high-temperature led to four new quaternary compounds, namely, Ga₂MoQ₂O₁₀ (Q = Se, Te), In₂Mo₂Se₂O₁₃(H₂O) and In₂MoTe₂O₁₀. Ga₂MoQ₂O₁₀ (Q = Se, Te) are isostructural and their structures feature a 3D network of gallium selenite/tellurite with 12-member ring tunnels along b-axis, the distorted MoO₆ octahedra are attached on the wall of the above tunnels. The structure of In₂Mo₂Se₂O₁₃(H₂O) features a new pillared-layered architecture composed of 2D indium(III) selenite layers that are interconnected by Mo₂O₁₀ dimers, forming 8-membered ring tunnels along the b-axis. The structure of In₂MoTe₂O₁₀ features a 2D indium oxide layer formed by corner- and edge-sharing InO₆ and InO₇ polyhedra with MoO₄ tetrahedra and TeO_n (n = 4, 5) polyhedra hanging on both sides of the layer, there are weak interlayer Te–O bonds of 2.512 Å. Results of optical diffuse reflectance spectrum measurements indicate that all four compounds are insulators, which are in agreement with results of band structure calculations based on DFT methods.