A quantum chemical study of the reactivity of the 7-5-5-7 topological defect, corresponding to a carbon ad-dimer, on a (5,5) armchair nanotube is reported. The energies indicate that the defect is more reactive by 46-70 kcal/mol towards adsorbates containing pi-bonds, such as C2H4, O-2, and O-3, than are pristine nanotubes or the Stone-Wales 5-7-7-5 defect. The enhanced reactivity of this defect was also obtained for other sizes and types of carbon nanotubes. (C) 2007 Elsevier B.V. All rights reserved.