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Experiments are reported, which allow us to quantify the near-surface cation diffusion in (001) oriented Fe_{3}O_{4} single crystals at temperatures between 470 and 770 K. Thin homoepitaxial films of magnetite, grown using isotopically labeled ^{57}Fe, were investigated by neutron reflectivity and time-of-flight secondary ion mass spectrometry. By heating the thin films in high vacuum to different temperatures for a well-defined time and determining the ^{57}Fe distribution along the surface normal, the diffusion lengths are obtained. For the investigated temperature range, diffusion constants of the order of 10^{−20} m^{2}/s are deduced. These results are important in view of near-surface mass transport induced by oxygen chemical potential differences occurring when magnetite is exposed to different gas atmospheres or by adsorbates.