Zeolitic material is obtained from fly ash both by direct conversion of the ash or from SiO2 extracts obtained from fly ash. This study focuses on determining the suitability of a fly ash for SiO2 extraction and for zeolite synthesis by direct conversion. The SiO2 extraction experiments from different fly ashes show that the main parameters governing the SiO2 extraction are: (a) a high bulk SiO2 content ( > 52%, for obtaining an extraction yield of 100 g SiO2 kg(-1)) in the starting fly ash, (b) a high proportion ( > 55%) of the bulk SiO2 present in the degradable glass matrix and the highly soluble opaline fraction, and (c) a high bulk SiO2/Al2O3 ratio ( > 2.0) of the fly ash. The results from experiments of zeolite synthesis by direct conversion demonstrate that the most important criteria for the selection of a fly ash for this process are: (a) a high content of Al2O3 and SiO2 ( > 65%) (b) a high glass content ( < 63%) and (c) relatively low SiO2/Al2O3 ratio ( < 2.0). Multivariate analysis confirms the importance of the above-mentioned variables and shows some additional variables that have influence on ash behaviour under alkaline conditions. It quantifies the use of those variables for determining the suitability of ashes for SiO2 extraction and zeolite synthesis and is able to distinguish between the two. (C) 2004 Society of Chemical Industry.