The amount of residual, or unburned, carbon in fly ash is an important concern in the design and operation of pulverized coal fired boilers. Char oxidation is the slowest step in the coal combustion process, and the rate at which this heterogeneous reaction proceeds has an important effect on the degree of carbon burnout. There is an extensive literature on char combustion kinetics based on data in the early and intermediate stages of carbon conversion. A critical fundamental question is whether the small fraction of the fuel carbon that passes unreacted through a boiler is representative of the char during the main portion of the combustion process. This article addresses that question through a detailed characterization of eight carbon-containing fly ash samples acquired from commercial scale combustion systems. The fly ash characterization included measurement of joint carbon-size distribution and determination of the combustion reactivity of the residual carbon. To minimize mineral matter interactions in the reactivity tests, the technique of incipient fluidization was developed for separation of carbon-rich extracts ( 75 wt% carbon) from the inorganic portion of the fly ash. Reactivity measurements were made at 1400-1800 K to represent conditions in pulverized coal fired boilers. Measurements were also made at 700-1100 K to minimize transport effects and to isolate the influence of char chemistry and microstructure. In both temperature regimes, the residual carbon extracts were significantly less reactive than chars extracted from a laboratory scale laminar flow reactor in the early to intermediate stages of combustion. It is concluded that the boiler environment deactivates chars, making high carbon burnout more difficult to achieve than is predicted by existing char combustion kinetic models that were developed from data on laboratory chars. Finally, the results are used to discuss potential char deactivation mechanisms, both thermal and oxidative, in coal fired boilers.