The N substitution reaction in a graphene sheet and in C nanotubes of various diameters was investigated by using the generalized tight-binding molecular dynamics method. The formation of a vacancy in a curved graphene sheet or a C nanotube was found to cause a curvature-dependent local reconstruction of the surface. Simulations and analysis showed that vacancy-mediated N substitution (rather than N chemisorption) was favored on the surface of nanotubes with diameters greater than 8nm. The predicted value of the critical minimum diameter for N incorporation was confirmed by experimental results on N-doped multi-walled nanotubes, with about 5at%N, prepared by thermal chemical vapor deposition. Vacancy-Mediated Mechanism of Nitrogen Substitution in Carbon Nanotubes. D.Srivastava, M.Menon, C.Daraio, S.Jin, B.Sadanadan, A.M.Rao: Physical Review B, 2004, 69[15], 153414 (4pp)