Three new series of Ho 2−x Er x Mo 4 O 15 (x=0.0–2.0), Ho 2−x Sm x Mo 4 O 15 (x=0.0–0.6) and Ho 2−x Ce x Mo 4 O 15 (x=0.0–0.25) solid solutions have been prepared successfully by solid-state reaction and studied by powder X-ray diffraction. All the XRD patterns of these molybdates can be indexed in monoclinic space group P2 1 /c. Lattice parameters a, b and c of Ho 2−x Ln x Mo 4 O 15 decrease linearly with increasing erbium content and increase with increasing samarium or cerium content. Thermal expansion behaviors of Ho 2−x Ln x Mo 4 O 15 have been investigated in the 25–500°C temperature range with high-temperature X-ray diffraction. The temperature dependence of Mo(2)–O14 interaction looks like to be responsible for their thermal expansion behaviors.