The equilibrium phase diagram between 0 and slightly above 50 mole% Bi 2O 3 in the Bi 2O 3MoO 3 system has been studied by differential thermogravimetric analysis (DTA) and X-ray diffraction measurements on fused mixtures and single crystals. The results confirm the existence of the four compounds α (Bi 2O 3·3MoO 3), β (Bi 2O 3·2MoO 3), γ (Bi 2O 3·MoO) 3 and ∊ (∼1.3Bi 2O 3·MoO 3) in the system. However, the phase diagram as well as the nature of melting of the α and γ were found disagreed with previous results. The γ compound melts incongruently at 947°C, whereas the α compound melts congruently at 662°C. The crystal class and lattice parameters of the compounds were determined based on the single crystal as well as powder pattern techniques. The results show that all four compounds have the monoclinic structure. The unit cell parameter of the β, γ, and ∊ compounds were found to be quite different from previously reported data. The lattice parameters obtained from X-ray analysis were also verified by density measurements of the single crystals. The polymorphism of the compounds was also investigated with single crystal samples. No polymorphic transformations for the α, β, and γ phases were detected in the work.