Temperature is one of the controlling factors determining the chemical structure of char. We employed advanced solid-state C-13 NMR techniques to characterize maple wood and its chars produced under wN(2) at temperatures from 300 to 700 degrees C. Our results indicated that 300 degrees C char was primarily composed of residues of biopolymers such as lignin and cellulose. Carbohydrates are completely lost for char prepared at 350 degrees C. At 400 degrees C, the char lost most of the ligno-cellulosic features and consisted predominantly of aromatic structures. By 500 degrees C, sp(3)-hybridized carbon had all but disappeared. Protonated aromatic carbons increased up to 400 degrees C chars but then decreased. Aromatic C-O groups decreased, whereas nonprotonated aromatic carbons, especially bridgehead carbons, increased as temperature increased. The minimum aromatic cluster sizes estimated from spectral analysis increased from 8 carbons in 300 degrees C char, to 20, 18, 40, 64, and 76 carbons, respectively, in 350 degrees C, 400 degrees C, 500 degrees C, 600 degrees C, and 700 degrees C chars. H-1-C-13 long-range dipolar dephasing displayed the same increasing trend of aromatic cluster sizes of wood chars with increasing temperature. We show for the first time quantitative changes of different aromatic C forms and aromatic cluster size as a function of heat treatment temperature.