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To better understand the transport of Mo and W in granitic melts and the formation mechanism of porphyry ore deposits, we have investigated the diffusivities of Mo and W in granitic melts with 0.04–5.1 wt% H2O at 1000–1600 °C and 1 GPa using a diffusion couple approach and a Mo saturation approach with Mo sheet serving as the source. The Mo and W diffusivities obtained from diffusion profiles measured by LA-ICP-MS can be described as: D Mo , anhy = 10 − 1.47 ± 0.73 exp [ − (387 ± 25) / R T ] , D W , anhy = 10 − 1.28 ± 1.05 exp [ − (396 ± 35) / R T ] , D Mo , 2.7 wt%H 2 O = 10 − 5.37 ± 0.52 exp [ − (211 ± 18) / R T ] , D Mo , 5.1 wt%H 2 O = 10 − 6.87 ± 0.69 exp [ − (133 ± 20) / R T ] , $\begin{align}& {{D}_{\text{Mo},\text{anhy}}}={{10}^{-1.47\pm 0.73}}\exp \left[ -{\left(387\pm 25 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{W},\text{anhy}}}={{10}^{-1.28\pm 1.05}}\exp \left[ -{\left(396\pm 35 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},2.7\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-5.37\pm 0.52}}\exp \left[ -{\left(211\pm 18 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},5.1\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-6.87\pm 0.69}}\exp \left[ -{\left(133\pm 20 \right)}/{\text{R}T}\; \right], \end{align}$ where D is diffusivity in m2/s (with the subscripts denoting water contents and "anhy" representing nominally anhydrous melt), R is the gas constant, T is the temperature in K, and the activation energies in the exponential are in kJ/mol. When the influence of H2O is incorporated, Mo diffusivity in granitic melts with <5.1 wt% H2O can be modeled as: log D Mo = − (1.94 ± 1.58) − (0.87 ± 0.36) w − [ (19341 ± 2784) − (2312 ± 620) w ] / T $log {{D}_{\text{Mo}}}=-\left(1.94\pm 1.58 \right)-\left(0.87\pm 0.36 \right)w-{\left[ \left(19341\pm 2784 \right)-\left(2312\pm 620 \right)w \right]}/{T}\;$ where w is H2O content in the melt in wt%. The diffusion behavior (low diffusivities, high activation energies, and strong H2O effects) of Mo and W indicates that they exist and diffuse in the melt in the form of hexavalent cations. Their low diffusivities imply that the bulk concentrations of Mo and W in exsolved hydrothermal fluid and those in the melt are probably not in equilibrium. However, because of the large fluid-melt partition coefficients of Mo and W, they can still be enriched in the hydrothermal fluid, although to a lesser extent than equilibrium partitioning would allow. Slow Mo and W diffusion can be a significant rate-limiting step for the formation of porphyry Mo/W deposits. [ABSTRACT FROM AUTHOR]