A new series of rare earth solid solutions of ErxDy2-xMo4O15 (x = 0.0-2.0) were successfully prepared. Their structures and thermal properties were studied by X-ray diffraction for temperature ranging from 25 to 500 degrees C. These solid solutions are found to crystallize in a monoclinic space group P2(1)/c. The lattice parameters a, b and c of ErxDy2-xMo4O15 decrease linearly with increasing erbium content. High-temperature XRD results show that these solid solutions show positive thermal expansion with rising temperature, very likely related to the weak bond Mo center dot center dot center dot O14 in ErxDy2-xMo4O15. To make compounds of Ln(2)Mo(4)O(15) family show negative thermal expansion, a possible way is to strengthen the weak Mo center dot center dot center dot O14 bond by shortening the Mo center dot center dot center dot O14 bond length. (c) 2007 Published by Elsevier B.V.