Mechanisms for NO desorption during reactions between O-2 and nitrogen-containing char have been studied using density functional theory at the B3LYP/6-31G(d) level. The chemisorption of O-2 on char surface was found to be barrierless and 440 kJ mol(-1) exothermic. A number of reaction pathways with barriers below 440 kJ mol(-1) were characterized. The chemisorption product could proceed directly to NO desorption with an energy barrier of 419 kJ mol(-1) or undergo a series of rearrangement reactions to release NO, in which the highest energy barrier was 360 kJ mol(-1). An oxygen atom could migrate along the edge of char if an adjacent active site was vacant. The overall migration reaction was 107 kJ mol(-1) exothermic. Transition state theory was used to calculate forward and reverse rate constants of intermediate reactions in the range of 1000-1800 K and two-parameter Arrhenius expressions were fitted. NO formation from nitrogen-containing char heterogeneous oxidation was dominated by indirect pathway consisting of a series of rearrangement reactions.