The speciation and molal formation quotients for the complexation of aluminum with sulphate were measured based on potentiometric and solubility experiments. Potentiometric titrations, utilizing a hydrogen-electrode concentration cell, were performed from 50 to 125°C at ionic strengths of 0.1, 0.3 and 1.0 molal in aqueous NaCl media. Two aluminum-sulphate species, AlSO4+ and Al(SO4)2-, were identified from the titration data and the formation quotients for these species were modeled by empirical equations to describe their temperature and ionic strength dependencies. Thermodynamic parameters for the complexation reactions were obtained by differentiating the empirical equations with respect to temperature. The thermodynamic quantities obtained for the formation of AlSO4+ at 50°C and infinite dilution are: logK1 = 3.7 ± 0.4, ΔH1°= -10 ± 30 kJ · mol-1, ΔS1°= 40 ± 100 J · K-1 · mol-1 and ΔC°(p 1) = 1900 ± 800 J · K-1 · mol-1; whereas the values for Al(SO4)2- are: logK2 = 5.6 ± 0.7, ΔH2°= 10 ± 50 kJ · mol-1, ΔS2°= 100 ± 100 J · K-1 · mol-1 and ΔC°(p 2) = 2800 ± 800 J · K-1 · mol-1. A solubility study, which was undertaken to verify the 50°C potentiometric data, was performed by reacting powdered gibbsite (Al(OH)3) with sulphate solutions at 10-3.5 and 10-4 molal H+, total sulphate concentrations from 0.005 to 0.080 molal, and 0.1 and 1.0 molal ionic strength in aqueous NaCl media. The results of the solubility study are in good agreement with the potentiometric data and establish that Al-sulphate complexation substantially enhances the equilibrium solubility of gibbsite.