Crystal structures and thermal expansion properties of Dy(2-x)Ln(x)Mo(4)O(15) (Ln = Ho (x = 0.0-2.0) and Sm (x = 0.0-0.4)) have been investigated by means of X-ray diffraction (XRD). Dy(2-x)Ln(x)Mo(4)O(15) (Ln = Ho and Sm) crystallizes in monoclinic lattice with space group P2(1)/c in the 25-500 degrees C temperature range. High-temperature XRD result reveals an anisotropic expansion in the lattice of Dy(2-x)Ln(x)Mo(4)O(15) with the expansion of a- and c-axes and contraction of b-axis. Despite the shrinkage of b-axis, the volume thermal expansion coefficient of Dy(2-x)Ln(x)Mo(4)O(15) is positive. The thermal expansion behavior of Dy(2-x)Ln(x)Mo(4)O(15) is basically related to the temperature-induced expansion of bond Dy(Ln)-O and Mo(2)-O14. (C) 2008 Elsevier B.V. All rights reserved.