Removal of Cr(III) from aqueous solution using Spirulina biomass was studied in a batch experiment. Spirulina biomass was characterised before and after Cr(III) sorption using FTIR, SEM and potentiometric titration. The effects of experimental parameters such as solution pH, contact time, biomass dose, initial Cr(III) concentration and temperature were studied. pH had significant influence on Cr(III) uptake capacity of biomass. The optimum equilibrium solution pH was found as 6.2. Biosorption efficiency decreased at higher temperature. A simple proton binding model was investigated for the quantitative determination of acid–base characteristics of Spirulina. The dissociation constants (pKa) of 1.19, 2.87 and 8.07 mol L−1 were estimated, which correspond to carboxylic, phosphatic and amine groups, respectively. A modified metal binding model was then tested, and Cr(III) binding constants (Kθ) of 80.9, 3.9 × 103 and 3.5 × 104 L mol−1 were found for carboxylic, phosphatic and amine groups, respectively.