The complexation of sulfate ions at the goethite-water interface has been studied by equilibrium measurements at 298.2 +/- 0.5 K and by infrared spectroscopy. A unified set of data from potentiometric titrations and batch adsorption experiments were evaluated according to an extended Constant Capacitance Model. The obtained equilibrium model consists of two outer-sphere complexes: = FeOH2+, SO42- (log beta(1,1,1(intr)(out)) = 8.3 +/- 0.03) and drop FeOH2+, HSO4- (log beta(2,1,1(intr)(out)) = 13.5 +/- 0.07) and an inner capacitance, C-1, of 4.5 and an outer capacitance, C-2, of 1.8. The outer-sphere model was confirmed by the infrared data and adsorption experiments at different ionic strengths. Furthermore, the infrared measurements provided direct evidence for protonation of the sulfate ions at the interface.