Bag filter (BF) fly ash from a hazardous waste incinerator located in Medellin, Colombia was characterized. Particle size distribution, chemical composition, metal loading. surface area, morphology, and chemical environment were assessed before and after fly ash extraction with toluene. Fly ash consists of low surface area platelets of SiO2 smaller than 0.5 mu m agglomerated in spheres between 20 and 100 mu m. High concentration of sodium chloride, carbon, and heavy metals such as Cu, Fe, Pb, Hg, Cd, Co and Mn are deposited over the fly ash surface. The carbon is oxidized and forms different structures such as amorphous carbon black, nano balls and more crystalline fullerenes like nano onions. The high concentration of dioxins, furans and dioxin-like PCBs (superior to 185 ng WHO-TEQ/g) is favored by oxidized carbon, chlorine and metals such as Cu and Fe on the shell of the particles. Before and after toluene extraction, fly ash samples presented similar morphology. However, after extraction their particle size increased while their surface area decreased by 35% and the carbon and metal contents decreased by 35% and 50%, respectively. (C) 2009 Elsevier B.V. All rights reserved.