This paper reports the results of a study to determine the effects of supplementary cementitious materials (SCMs) on the chloride binding of portland cement pastes. The results show that SCMs with significant quantities of alumina increase the binding capacity of cement paste. Pastes with metakaolin (45% Al 2O 3) showed the greatest chloride binding capacity and pastes with silica fume (< 0.5% Al 2O 3) showed the least binding. The chemical binding in solutions of high chloride concentration is mainly attributed to the formation of Friedel's salt, C 3A·CaCl 2·10H 2O. When pastes originally exposed to high chloride concentrations are subsequently exposed to chloride-free solution, a portion of the bound chloride is released, but a significant portion remains irreversibly bound. There is some evidence that Friedel's salt may partially convert to Kuzel's salt, C 3A·(0.5CaCl 2)(0.5CaSO 4)·12H 2O, under these conditions. The binding relationships were best described by the Freundlich isotherm and binding coefficients are given for all the binders tested.