Chloride binding capacity of hydrated C3A pastes exposed to chloride solutions was investigated based on equilibrium method and the influences of chloride concentration, pH value and Ca2+ of exposure solutions were studied. The results show the product from C3A hydration, tricalcium aluminate hexahydrate(C3AH6), can bind chloride in alkaline solution to yield two types of solid solutions between the chloride and hydroxide end members, and the structure of which shows similarities with α Fs M(monoclinic low temperature structure) and β Fs R(rhombohedral high temperature structure) separately. The pH value of chloride solutions affects the type of solid solutions formed. With increasing chloride concentration, although the amount of bound chlorides and solid solution formed increases, the ratio of bound to total chlorides decreases, and the pH value of the chloride solutions increases. The reduction in pH value of exposure solutions decreases chloride binding capacity by less formation of solid solutions. Although Ca2+ leads to the decreasing of pH value and solid solution formed, the chloride binding capacity of hydrated C3A paste increases.