Semiempirical MNDO and density functional theory (B3LYP/3-21G) were used to examine the relative stability of the isomers of successive BN-substituted fullerenes C60-2x(BN)x, where x = 1−24. Doing so established certain rules of successive BN substitution in cagelike carbon materials. HOMO−LUMO gaps were also estimated for the entire series. It was found that heterofullerenes with less than 40% BN substitution have smaller band gaps than does semiconducting C60.