Two Greek lignite fly ashes (FAs), with relatively different mineralogy and chemistry, were examined for their ability of giving rise to zeolitic material under alkaline activation. The two FAs were collected from Ptolemais (PFA) and Megalopolis (MFA) Lignite Power Plants and underwent a hydrothermal treatment utilizing sodium hydroxide solution 1 M, at a temperature of 90 degrees C for 24 h. Four different FA/NaOH ratios (25, 50, 100 and 150 g L-1) were examined for each of the FAs. Experimental products were characterized by means of X-ray diffraction analysis and scanning electron microscopy and also evaluated in terms of their cation exchange capacity (CEC), specific surface area (SSA), specific gravity (SG) and pH. Results showed relatively similar behavior for the two FAs and a significant mono-mineral zeolite formation for FA/NaOH = 50 a L-1. For the particular ratio, NaP1 (Gobbinsite, GIS-type), and NaP zeolites were formed from aluminosilicate glass digestion. For lower FA/NaOH ratios, the excess of NaOH seems to be prohibitive for zeolite growth, while for the highest solid/liquid ratios, the NaOH quantity seems to be insufficient for the effective activation of the FAs. The values of CEC and SSA were also higher for FA/NaOH = 50 g L-1, with the MFA presenting the best results. The authors' current work focuses on testing the products of the aforementioned zeolitization process for their potential of retaining heavy metals from a contaminated soil. (C) 2005 Published by Elsevier Ltd.