We used density functional theory calculations to probe the chemical reactivity of graphene and single-wall carbon nanotubes (CNTs) toward the small molecules O-2, H-2, N-2, C2H2, CO, and CO2, We found that there is a threshold CNT size below which C2H2, and CO, typical feedstock precursors for CNT growth, become trapped in decorated hillock-like defects on the side walls of CNTs. We also found that O-2, H-2, and CO2 can etch isolated C adatoms and C adatom pairs. These processes play a role not only in the growth of CNTs, but also in the postgrowth evolution of defects on CNTs through exposure to typical ambient gases.