The solubility of H2O in silicate melt drops substantially with decreasing pressure, so that a magma initially containing several weight percent H2O in a crustal magma reservoir is left with only a few thousand parts per million following ascent and eruption at the Earth’s surface. This rapid release of volatiles makes determining the pre-eruptive H2O contents of magmas very difficult. Olivine-hosted melt inclusions are thought to retain their H2O because they are protected from decompression by the strength of the host crystal, and pre-eruptive concentrations obtained from melt inclusions have been used to both estimate the amount of H2O in the upper mantle and investigate its role in the melt generation process. The greatest uncertainty involved in constraining upper mantle conditions from melt inclusions is the potential for rapid diffusive loss or gain of H+ (protons) through the host olivine. Here we present results from hydration and dehydration experiments that demonstrate that, contrary to the widely held view, H2O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H+ through the host olivine are possible on very short time scales. We also show that the Fe3+/ΣFe of an olivine-hosted melt inclusion maintains equilibrium with the external environment via diffusion of point defects through the host olivine. Our results demonstrate that, while pre-eruptive H2O and Fe3+/ΣFe can be reliably estimated, olivine-hosted melt inclusions do not necessarily retain a record of the H2O and O2 fugacity conditions at which they formed. High-H2O melt inclusions are particularly susceptible to diffusive dehydration, and therefore are not reliable proxies for the state of the upper mantle.