To clarify the mechanism of zeolite synthesis from coal fly ash, the hydrothermal. reaction was carried out in various alkali solutions, Zeolite was synthesized in an 800-cm(3) autoclave under the condition of 393 K and 100 g/400 cm(3) of solid-liquid ratio. The changes in various physical and chemical properties, such as crystal structure, surface structure and cation exchange capacity, of the obtained zeolites and the dissolved amount of Si4+ and Al3+ in alkali solution were investigated during the hydrothermal reaction. The mechanism of zeolite crystallization and the role of alkali solution on the synthesis reaction were considered. Zeolite P and chabazite are mainly synthesized as the crystal type of zeolite from coal fly ash. There exist three steps in alkali hydrothermal reaction of zeolite synthesis: the dissolution step of Si4+ and Al3+ in coal fly ash, the condensation step of silicate and aluminate ions in alkali solution to make aluminosilicate gel, and the crystallization step of aluminosilicate gel to make zeolite crystal. The OH- in alkali solution remark-ably contributes to the dissolution step of Si4+ and Al3+ in coal fly ash, while Na+ in alkali solution makes a contribution to the crystallization step of zeolite P. This zeolite has the tendency to capture K+ selectively in the cation exchange site. (C) 2002 Elsevier Science B.V. All rights reserved.