A new series of solid solutions Y2W3-x Mo (x) O-12 (0.5 a parts per thousand currency sign x a parts per thousand currency sign 2.5) were successfully synthesized by the solid state method. Their crystal structure and negative thermal expansion properties were studied using high-temperature X-ray powder diffraction and the Rietveld method. All samples of rare earth tungstates and molybdates were found to crystallize in the same orthorhombic structure with space group Pnca, and show the negative thermal expansion phenomena related to transverse vibration of bridging oxygen atoms in the structure. Thermal expansion coefficients (TEC) of Y2W3-x Mo (x) O-12 were determined as -16.2 x 10(-6) K-1 for x = 0.5 and -16.5 x 10(-6) K-1 for x = 2.5 in the identical temperature range of 200-800 A degrees C. High-temperature XRD data and bond length analysis suggest that the difference between W-O and Mo-O bond is responsible for the change of TECs after the element substitution in this series of solid solutions.