In order to clarify the possibility of utilization of blast furnace slag containing chlorine, chlorine bearing phases in the synthetic CaO-SiO₂-Al₂O₃-Cl slags were characterized and their dissolution behavior was investigated. The synthetic slags with various crystalinity were identified by XRD and SEM-EDX. Chlorine was found to condense in the amorphous phase and 9CaO･6Al₂O₃･5SiO₂･CaCl₂ whereas it was hardly detected in the other crystallized phase, such as CaO･SiO₂, 2CaO･Al₂O₃･SiO₂, CaO･Al₂O₃･2SiO₂. The dissolution behavior of chlorine into aqueous solution was studied to grasp the stability of chlorine in the slag. Experimental time (Shaking time) and pH dependences on the solute dissolved from the two main chlorine containing phase, amorphous phase and 9CaO･6Al₂O₃･5SiO₂･CaCl₂, were measured. The amount of chlorine dissolved from amorphous phase was smaller than that from 9CaO･6Al₂O₃･5SiO₂･CaCl₂ (Fig. 1). The amount of solute dissolved from the slag tended to increase with the fraction of crystallization (Fig. 2). Furthermore, the crystal phases in blast furnace slags were identified by XRD and SEM-EDX, and there were two main crystal phases, melilite (Ca₂(Al, Mg)(Si, Al)₂O₇) and 9CaO･6SiO₂･CaCl₂ in it. The similarity between synthetic slag and BF slag was that the amount of Cl dissolved from slag increased with crystallizing.