A western Kentucky power plant conducted a series of test bums with coal + tire-derived fuel (tdf) and coal + tire-derived fuel + petroleum coke blends. Collections of fuel, fly ash, and bottom ash/slag were made from the cyclone-fired unit under four fuel combinations: coal, coal + ca. 1% tdf, coal + ca. 3% tdf, and coal + ca. 3% tdf + petroleum coke. Fly ash carbons derived from the three fuel types can be distinguished, allowing an assessment of the impact of co-combustion on fly ash quality. While certain aspects of the ash chemistry are distinctive, Zn increasing in tdf-derived fly ash and Ni and V increasing in petroleum coke-derived fly ash, changes in the coal source between sampling dates complicate the assessment of the chemistry.