Structures, thermal expansion properties and phase transitions of Er x Fe 2−x (MoO 4 ) 3 (0.0≤x≤2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er 3+ for Fe 3+ induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe 2 (MoO 4 ) 3 , as well as the orthorhombic Er x Fe 2−x (MoO 4 ) 3 (x≤0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er x Fe 2−x (MoO 4 ) 3 (x=0.6–2.0) decrease with increasing content of Er 3+ and for x≥1.0, compounds Er x Fe 2−x (MoO 4 ) 3 show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of −0.60×10 −6 /°C in Er 1.0 Fe 1.0 (MoO 4 ) 3 is observed in the temperature range of 180–400°C, and zero thermal expansion is observed in Er 0.8 Fe 1.2 (MoO 4 ) 3 in the temperature range of 350–450°C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er x Fe 2−x (MoO 4 ) 3 compounds, with negative thermal expansion coefficients along the a axes.