【Objective】As the most dynamic and bioavailable fraction of the soil carbon pool, dissolved organic matter (DOM) plays important roles in a wide range of biogeochemical processes in the environment. The interaction of soil minerals with DOM would induce structural fractionation of the DOM at the mineral/water interface, thereby influencing long-term preservation and biogeochemical processes of the DOM in the soil. Ferrihydrite, a poorly crystalline iron oxide, is known to be highly reactive to DOM in the soil owing to its high specific surface area and abundant reactive binding sites. Widespread in the environment, it can associate with DOM through either adsorption or co-precipitation. However, so far, few studies have been reported on structural fractionation of DOM at the ferrihydrite/water interface induced by co-precipitation at the molecular level. 【Method】To explore molecular fractionation of DOM at the ferrihydrite/water interface during its co-precipitation process, this study prepared ferrihydrite-DOM complex separately in solutions different in C/Fe ratio via co-precipitation and adopted the technology of combining ultraviolet (UV) spectrum with electrospray ionization Fourier transform ion cyclotron resonant mass spectrometry (ESI-FT-ICR-MS) to probe changes in DOM composition after co-precipitation ended. Species of carbon and Fe in the complex were determined with the aid of X-ray photoelectron spectroscopy (XPS) and Fe K edge XAS.【Result】Results show that Fe in the complex existed mainly in the form of ferrihydrite, of which the proportion gradually decreased from 95.9% to 68.0% with the increase in initial C/Fe ratio of the solution. UV and ESI-FT-ICR-MS analysis collectively revealed that during the co-precipitation process, ferrihydrite fixed in priority aromatic components high in molecular weight and rich in oxygen (mainly combustion-derived condensed polycyclic aromatic- and vascular plant-derived polyphenols-like substances) in the DOM, leaving aliphatic components in the solution. The lower the solution in C/Fe ratio, the higher the molecular fractionation in degree. This feature is basically consistent with what has been reported about the fractionation induced by adsorption, which indicates that no matter whether in the process of adsorption or co-precipitation, ferrihydrite tends to fix DOM components high in molecular weight and aromaticity, thus leading to changes in chemical composition of the DOM as well as surface properties of the ferrihydrite, which will in turn affect sorption, transportation and transformation processes of the contaminants therein. In addition, this study was the first to find that ferrihydrite varied dynamically with the reaction in time in selectivity to structure of the DOM, that is, combustion-derived condensed polycyclic aromatics were preferentially fixed in the ferrihydrite-DOM complex, and with the reaction going on, vascular plant-derived polyphenols-like substances followed.【Conclusion】The findings in this study are expected to be able to help understand in depth the mechanism of molecular fractionation that affects geochemical behavior of DOM in the environment via co-precipitation during the process of ferrihydrite formation.