This study focuses on the mobility of water-soluble major and trace elements in six Spanish By ashes using room temperature open and closed and heated (90 degreesC) closed leaching systems in an attempt to reduce the content of undesirable species in coal combustion by-products to increase their potential applications. The leaching trends of the trace impurities are consistent with the dissolution of small solid particles or coatings on the surface of the ash solid phases rather than with the dissolution of a homogeneous glass phase. The leaching rate of the different trace impurities can be arranged in decreasing order as B greater than or equal to Mo greater than or equal to Se > Li > Sr greater than or equal to Cr greater than or equal to As = Ba = Cd = V > Sn > Rb = Zn greater than or equal to Cu = Ni = Ph > U > Co > Mn. Although several elements showed increased mobility in the room temperature open leaching system with respect to the closed extraction, the differences obtained for major impurities were not significant with the exception of the extraction ratios obtained for As and V in high lime By ashes. The extraction ratios were higher for the heated leaching than those obtained for room temperature extraction for the elements Al. Si, K, Na, Ba, Cr, Rb, Sr and V. However, the mobility of major impurities (Ca and Fe) and heavy metals did not increase considerably in the heated extraction (generally <30%). (C) 2001 Elsevier Science Ltd. All rights reserved.