Phase pure, fine ZrW2−δMoδO8 powders with 0.00≤δ≤0.91 were synthesized by a hydrothermal route. As-synthesized particles exhibited a chemical composition consistent with zirconium tungstate–molybdate hydroxide dihydrate (Zr(W,Mo)2 O7(OH)2·2H2O). Transmission electron microscopy revealed a rod-like morphology with each particle being a single crystal. X-ray diffraction patterns revealed that the crystallite size as well as the disordered β phase increased with increasing Mo substitution for W. Fourier transform infrared spectroscopy showed characteristic stretching frequencies of Zr–O, (W,Mo)–O tetrahedral, as well as OH stretch. Differential scanning calorimetry showed that the order–disorder transition temperature decreases linearly with Mo content.