The object of this study is to investigate how the cooling rate and the basicity during the vitrification process govern the chemical stability and acid resistance of slags. In this experiment, the incineration fly ashes with various basicities were vitrified at 1450 degrees C and cooled down by air cooling and water quenching, respectively. The amorphous volume fractions (AVF) in slags were estimated by X-ray diffraction analysis with the addition of an internal standard. Scanning electron microscopy was applied to qualitatively examine the microstructures of the original and the acid-immersed slags. It was verified that the addition of SiO(2) and water quenching both significantly affected the crystalline characteristics of the slag. When the basicity was >0.990, the AVF of slags was approximately equal regardless of the cooling rate. When the basicity was <0.674, water quenching greatly enhanced the formation of glassy amorphous phase and the immobilization of metals in slags. In contrast, a lower basicity (<0.511) is required if one wants to vitrify fly ash into slags and cool it down by air. As a whole, the glassy amorphous structure presented better chemical resistance than the crystalline structure to decomposition by an acid.