A study using ab initio MO calculations and frontier orbital theory has been performed to investigate the effect of substitutional boron on the electronic structure and reactivity of eight-ring carbon model structures. This theoretical analysis confirmed that boron substitution in the carbon lattice can result in two opposite effects on carbon oxidation: catalysis and inhibition. Boron substitution was found to decrease the global cluster stability and to affect the local reactivity of its edge sites. For a zigzag cluster, the reactivity of carbon active sites may be increased or decreased by boron substitution and the exact effect appears to be dependent on substituent position: in general, the reactivity of unsaturated edge sites decreases, but substitution at certain basal-plane sites may increase the reactivity of some active sites which in turn suggests a catalytic effect. For an armchair cluster, boron substitution increases the reactivity of one or more armchair edge sites. Single atom substitution in the zigzag cluster may result in thermodynamically favorable or unfavorable O-2 chemisorption; the exact effect was found to be site-dependent. It also increases the energy barrier for CO desorption. Such an intriguing dual effect provides an explanation for the experimentally observed conflicting effects of boron doping in carbon oxidation.