We have determined, through thermogravimetric analyses (TGA) of seventy fly ash samples obtained from diverse coals and boilers, that loss-on-ignition (LOI) consistently overestimates unburned carbon in fly ash. Experimental results have shown that LOI overestimated unburned carbon by at least 20% in 44% of the fly ashes tested. Although some of these errors may be associated with previously identified thermal decomposition of minerals in fly ash, volatile organic compounds appear to play a role for some of the fly ashes evaluated in this study. This discovery has implications in the use of LOI to calculate combustion losses and monitor fly ash quality by power plant managers. We propose an alternative method to measure unburned carbon based on TGA.