By fusion with sodium hydroxide followed by a hydrothermal reaction, fly ash and Al-enriched fly ash were converted into Na-X and Na-A zeolites, respectively. The authentic Na-X, Na-A and fly ash zeolites as well as their amorphous precursors have been characterized by IR, Si-29 and Al-27 MAS NMR, XPS/AES, TG, and comparative ion-exchange studies of Cs and K with Na in zeolite samples. It appears that the same structural unit with a terminal OH, such as a sodalite unit, was formed in the induction period of the hydrothermal reaction for Na-X and Na-A, and then cross-linked through D4R and D6R external linkages to build up the zeolite framework of Na-A and Na-X, respectively.