Surface complexation of inorganic mercury(II) and monomethyl mercury (CH3Hg+) onto a hydrous goethite (alpha-FeOOH) surface has been studied. The ph (-log[H+]) has been varied within the range 2.7 < -log[H+] < 9, at 298 K, using a 0.1 mol dm(-3) NaNO3 medium. Low total mercury concentrations(approximate to 1 mu mol dm(-3)) were used. Inorganic mercury and methyl mercury content in the water phase, in the goethite phase, and adsorbed on the vessel walls was determined by GFAAS and GC-coupled MIP-AES. The inorganic mercury distribution was evaluated, using a previously determined surface complexation model, based on the constant capacitance concept. This model was derived using about 10(3) times higher total mercury concentrations. In the presence of goethite, approximately 80% of both methyl mercury and inorganic mercury(II) was recovered in the different phases. The extractable fraction adsorbed to the vessel walls was found to be low. About 20% of the inorganic mercury added remained soluble in the presence of goethite. This is close to the predicted value, using a previously determined model derived in the mmol dm(-3) range. This implies that the concept of specific high affinity sites is questionable in the case of near trace concentrations of Hg(II). Methyl mercury was found to be weakly coordinated, with about 15% adsorbed on the goethite surface. Furthermore, a model calculation describing the surface complexation of methyl mercury on goethite was performed. The experimental data could best be explained by the formation of the surface complexes =FeOHHgCH3+ and =FeOHgCH3.