The calcium binding capacity of rice bran was evaluated by using adsorption isotherms at several pH values. Adsorption increased with pH and was associated with an increase in negative surface charges in plant fibres. At pH 4.4, the release of endogenous calcium was the main process, while adsorption determined Ca availability at higher pH. The Ca concentrations, at which neither adsorption nor release occurs were as follows: 7.86 × 10−3 m (pH 4.4), 4.14 × 10−3 m (pH 5.0), 2.31 × 10−3 m (pH 6.0) and 0.85 × 10−3 m (pH 7.5). When using NaCl as the electrolyte, the results showed that competition of Na+–Ca2+ for binding sites seemed to be the determinant of the rate of Ca adsorption. Negative electric charges in bran are produced by ionization reactions which depend on solution pH and were evaluated by using the Gouy–Chapman double-layer model. The value of pH0, at which the surface charge is zero was found to be 5.5.