The objective of this study is to investigate how cooling rate and basicity in a vitrification process govern the crystalline characteristics of slags. In this experiment, the incineration fly ash mixtures with various SiO2 addition ratios were vitrified at 1450 degrees C and cooled down separately by air or water. Different thermal analysis, scanning electron microscopy and X-ray diffraction analysis with an internal standard addition were applied to investigate the crystalline characteristics of slags. The microanalytical mapping images showed that water quenching and the addition Of SiO2, both enhanced the glassy amorphous phase to distribute more uniformly in slags. Addition of SiO2 would lower the melting temperature of fly ash mixtures and retard the formation of crystalline phases in slags. When the basicity (mass ratio of CaO to SiO2 before vitrification) was >0.990, the profiles of crystalline phases in slags with equal basicity were similar no matter how they were cooled. However, when the basicity <0.674, water quenching greatly enhanced the formation of the glassy amorphous phase in slags. For air cooled slags, an even lower basicity (<0.511) is required to vitrify fly ash well. (C) 2007 Elsevier B.V. All rights reserved.