Carbon burnout, measured as percentage combustibles remaining, was determined for 16 world coals on a 1 MW p.f. combustion rig. The combustibles-remaining figures were correlated with parameters such as ash yield, carbon content, fuel ratio, rank, reactive macerals content and a reactivity parameter determined from a grey-scale histogram obtained by image analysis of the coals. The reactivity parameter was found to give a good correlation with combustibles remaining, far superior to the correlation given by any other parameter. Char samples collected at various points in the combustion rig confirmed that the intrinsic reactivity of char decreases during the combustion process and that the morphology of the char produced during the very early stages of combustion has a much greater effect on carbon burnout than intrinsic reactivity does.