Productively reusing the waste residuals from energy production is an essential component in sustainable disposal and management of energy related waste. In the US, the world's second largest producer of coal next to China, over 118 million tons of coal combustion by-products are produced each year; only 44% of which are productively reused. In recent decades, advances in lowering CO2, SOX, and NOX emissions from coal combustion have changed the characteristics of the solid coal combustion products. In particular, the residual carbon content of fly ash generated at many US coalfired power plants has increased, and has become more heterogeneous. Additionally, utilities are exploring the use of pure biomass as a fuel source, which results in a fly ash that is generated from a purely organic source. While much research effort has been devoted to understanding the properties and potential productive reuse alternatives for coal combustion products, relatively little research has been done on the byproducts from biomass combustion or co-combustion. In this study, high carbon content coal ash, co-fired coal/biomass ash, and pure biomass ash from several US power plants were investigated. The ashes were characterized using a number of physical and chemical analysis techniques, including: scanning electron microscopy, laser diffraction, organic carbon content, nitrogen adsorption surface area, proximate and ultimate, X-ray fluorescence, and X-ray diffraction analysis. Results showed little physical, chemical, and mineralogical differentiation between coal ash and coal co-fired with biomass ash. However, the pure biomass ashes investigated in this study, showed significantly lower specific gravity, and primary oxide content, as well as coarser particle size distribution, higher residual carbon, higher heating value, and much higher specific surface area, when compared to the coal and co-fired ashes. These results have important implications on the potential for productive reuse of these waste materials. (C) 2013 Elsevier Ltd. All rights reserved.