One of the most important indices for evaluating the combustion efficiencies of boilers, as well as the commercial value of the produced fly ash, is the unburned carbon in fly ash. The most common method currently used by combustion engineers to estimate the amount of unburned carbon in fly ash is to equate it to the dry loss on ignition (LOI) value. There seems to be no reported systematic study linking LOI values with the true carbon content of ashes and combustion residues. In this study, the LOI values for 35 combustion residues were determined at 500, 750, and 950 degrees C, using a macro thermogravimetric analyzer. The carbon contents of the combustion residues and the residues from the LOI determinations were then measured. For the samples in this study, it was determined that temperatures of > 790 degrees C should be used to achieve complete carbon burnoff. For low-percentage-carbon combustion residues, there is very poor agreement between the unburned carbon contents and the LOI values. This is especially true if the samples are exposed to the atmosphere for extended periods of time, because the combustion residues readily absorb moisture and acidic gases. For high-percentage-carbon combustion residues, there is good agreement between the unburned carbon and the LOI values, especially if the residues are relatively fresh.