This study focuses on the behavior of major and trace water-soluble impurities in fly ash during leaching processes. The objective is to assess potential decontamination procedures in order to reduce the content of undesirable species in coal combustion by-products and to increase their potential industrial applications. Fourteen flyashes were selected for this study. Major impurities considered were lime and portlandite, sulfate, and some potentially hazardous trace elements such as As, Ba, Be, Cd, Co, Cr, Cu, Ge, Mn, Mo, Ni, Pb, Se,Th, U, V,and Zn.Water was used for the extraction experiments,with asolvent/fly ash ratio of 50 mL/g. An hydrite was completely dissolved in all fly ashes studied, except the Cercs and Escatron flyashes. Lime and portlandite were completely dissolved or carbonated from all flyashes with exception of the Cercs flyash. Bulk calcium contents were reduced from 9% to 18% in all fly ashes with the exception of the Compostilla and Espiel fly ashes (4-5%). From 17% to 79% ofthe sulfur was extracted from the bulk content. The results show a relatively higher extraction for S, Se, As, B, Mo, and Ca, and an intermediate extraction for As, Ge, Cd, Li, Sr, Na, V, Cr, Sn, P, and Ba. The results suggest that the alkalinityis induced during the extraction mainly by free lime. The high alkalinity accounts for the high Al and Si concentrations in the leachates and the relatively low mobility of sometrace elements, with the exception of As, B, Mo, and Se.