A new hydration model of OPC [1, 2] was used in this study. The quantities of hydration products were calculated for saturated state hydration and for samples that have been dried to 11% rh after hydration. The chloride binding of several hydrated phases (C-S-H, monosulfate, hydroxy-AFm, AFt, CH, and Friedel's salt) has been studied and estimated in relation to the chloride content of the pore solution. Combining two new isotherms, for C-S-H and AFm chloride binding in hardened cement paste, and the computed quantities of all hydration products, a new formula for the chloride binding capacity of OPC pastes has been deduced. The new model is based on the chloride binding abilities of each hydrated phase, and therefore can distinguish between their contributions at different free chloride concentrations. When compared to experimental data, the results obtained using this new chloride binding isotherm have proven to accurately describe the chloride binding capacity of OPC pastes, results being within 12% of the experimental values for the whole range of free chloride concentrations considered, of up to 3 M.