Evidence is increasing for the mobility and bioavailability of aqueous mercury(II) species being related to the interactions of mercury with dissolved organic matter (DOM). Here, we assess the relative roles of the mineral surface and DOM in controlling mercury(II) uptake at the muscovite (001)-solution interface using interface-specific X-ray reflectivity combined with element-specific resonant anomalous X-ray reflectivity. Experiments were performed with single crystals of muscovite and solutions of 100 mg/kg Elliott Soil Fulvic Acid II and 0.5-1 x 10(-3) mol/kg Hg(NO3)2 at pH 2-12 Mercury(II) adsorbed from a 1 x 10(-3) mol/kg Hg(II) solution at pH 2 without fulvic acid (FA) as inner- and outer-sphere complexes that compensated 55(4)% of the permanent negative charge of the muscovite surface. The remaining charge presumably was compensated by hydronium. The enhanced uptake of Hg(II) (compensating 128% of the muscovite surface charge) and FA (43% more adsorbed compared to the amount from a similar solution without Hg), along with a broader distribution of Hg(II) at the interface, occurred by adsorption from a premixed solution of 1 x 10(-3) mol/kg Hg(NO3)2 and 100 mg/kg FA at pH 2. Adsorption of Hg(II) and FA, likely as complexes, decreased significantly as pH increased from 3.7 to 12 in solutions of 0.5 x 10(-3) mol/kg Hg(NO3)2 and 100 mg/kg FA. Preadsorbed FA molecules provide different binding environments and stability for Hg(II) than dissolved FA, which may be attributed to conformational differences, fractionation, or kinetic effects in the presence of the mineral surface, at least at these relatively high concentrations of aqueous Hg(II).