To elucidate the nature of element exchange between crystals via the mediation of a fluid phase, the mechanism and kinetics of the exchange reaction: 2Fe(Metal) + Mg2SiO4(Ol) = 2 Mg(Gas) + Fe2SiO4(Ol) were studied through a set of Knudsen cell mass spectrometry experiments. The reaction progress was monitored in situ by measuring vapor pressures of individual species (mainly Mg, SiO, and Fe) during experiments as well as by measurement of compositional profiles in crystals (Fe-rich rims) after the anneals. The experimental observations were reproduced by a numerical model which accounted for compositionally dependent diffusion in olivine, surface-controlled evaporation/condensation reactions (moving boundaries) and transport out of the system (open system behavior). The main finding of the study is that the composition (Fe/Mg ratio) at the surface of olivine varies with time at constant temperature, i.e., the final surface composition was not achieved instantaneously compared to the diffusion time scale, even during exchange with a vapor phase at high temperatures. The low concentration of Mg in the gas phase explains this behavior. On the basis of these observations a simple general model for element exchange between a solid and a fluid phase could be formulated and analytically solved. A single parameter may be defined combining thermodynamic and kinetic factors. The value of this parameter demarcates three domains: (1) disequilibrium at the surface, (2) equilibrium at the surface, and (3) no observable reaction, i.e., frozen system. It is shown that all three classes of behavior may be observed in natural fluids such as an ideal gas, aqueous solutions, and silicate melts.