In order to evaluate the effect of H on the pressure and transition interval of the olivine-wadsleyite transformation, synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400¿C. Under hydrous, near-saturated conditions, the pressure of the transition is 1.0 ¿ 0.5 GPa lower than under anhydrous conditions. In addition, the two-phase interval appears to broaden from about 0.4 GPa (12 km) in the anhydrous system to as much as 1.3 GPa (40 km) in the hydrous system. Under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by H diffusion. Extrapolating previously measured diffusion coefficients for H in olivine and in liquids, it appears that H diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite.