This study investigated the encapsulation of metals in vitrified slags using a sequential extraction procedure and how Al ions and slag structure affected the encapsulation of heavy metals. It is found that the substitution of Al ions for Si ions weakened the encapsulation of the glass matrix due to the relatively weaker single bond strength of Al-O. In addition, the substitution also governed the phase distribution of metals that tended to stay slags. In comparison to air cooled slags, water quenched slags were more amorphous and offered relatively better encapsulation of metals. These findings were also supported by the SEM observation and XRD analysis. Although Al2O3 may connect the non-bridging oxygen and polymerize the slag structure, the excess addition of Al2O3 reduced the metal encapsulation and acid resistance of slags. (C) 2009 Elsevier B.V. All rights reserved.