This study has been undertaken to determine the extent of structural ordering and thermal deactivation which can occur when coal chars are heated to high temperatures. The chars have been prepared using the captive-sample wire-mesh reactor at temperatures up to 2200 degrees C at very short hold times (500 ms), and the microstructures have been examined using high-resolution transmission electron microscopy (HRTEM). The char structures have been compared to those of natural graphites. Regions within the chars have been observed to show a similar degree of ordering to that present in the graphites. The reactivities for oxidation of the high temperature chars and graphites have been determined by TGA methods and compared to the reactivity of chars prepared at lower temperatures. The structural ordering and reactivity data indicates that thermal deactivation may account for the low reactivity of a proportion of the char present in utility boiler fly ash.