Eight commercial fly ash samples including two from petroleum coke/coal co-firing were characterized to determine the origin, structure, and properties of the unburned carbon. Because of its relevance to ash utilization, the characterization included measurements of adsorptivity toward concrete surfactants, and its dependence on carbon form, particle size, and contact time. The coal/coke co-firing ashes are observed to possess unique optical and adsorptive properties which are related to the size, density, and internal porosity of the coke-derived residual char.