We addressed here, by means of electrospray ionization mass spectrometry (ESI-MS) with ultrahigh resolution, the molecular level fractionation of a reference fulvic acid (SRFA) during its sorption at an alumina surface, taken as a model for surfaces of natural aluminum oxide hydrates. Examination of ESI-MS spectra of a native SRFA solution and of supernatants collected in sorption experiments at acidic pH showed that the ∼5700 compounds identified in the native solution were partitioned between the solution and alumina surface to quite varying degrees. Compounds showing the highest affinity for the surface were aromatic compounds with multiple oxygenated functionalities, polycyclic aromatic compounds depleted of hydrogen and carrying few oxygenated groups, and aliphatic compounds with very high O/C values, highlighting the fact that SRFA constituents were sorbed mainly via chemical sorption involving their oxygenated functionalities. We observed an inverse correlation between the degree of sorption of a molecule within a CH2 series and its number of CH2 groups and a positive correlation between the degree of sorption and the number of CO2 groups in a COO series, which was remarkable. These correlations provide evidence at the molecular scale that molecule acidity is the key parameter governing fulvic acid (FA) sorptive fractionation, and they are useful for predicting sorption of FA at a natural oxide surface.