This paper reports about changes in mobility of hazardous elements contained in coal during combustion at a power plant in Santa Catarina (Brazil) and the environmental impact potential of ashes. Total and mobile element concentrations were determined by digestion and sequential extraction. Comparison of results within the mobile fraction showed that after combustion, oxidizable elements bound to organic matter and sulfides in coal were mostly transformed into elements easily soluble in water or slightly acidic conditions and, hence, most readily bioaccumulative in the environment in ashes, mainly U, Cr, and As. Capacity of ashes as a source of mobile pollutants was quantified by combining sequential extraction and annual production. Just considering the easily soluble fraction, coal ashes could leach up to 839 tons of Al, 144 tons of Fe, 100 tons of Mn, 4.6 tons of Zn, 3.1 tons of Cr, 1.7 tons of As, 1.5 tons of Cu, 490 kg of U, and 20 kg of Pb every year. Bottom ashes are disposed of in landfill sites close to the plant. Fly ashes are recycled as construction material. Diagnostic processes do not consider checking for these highly mobile hazardous elements. Hence, uncontrolled dumping and use of these by-products may pose significant risks to environment and human health. © 2011 Elsevier Ltd. All rights reserved.