Low-energy irradiation-induced conductivity decrease of a single-wall carbon nanotube has been well established experimentally. However, its origin is still controversial. Irradiation effects on suspended single-wall carbon nanotubes, which are much less affected by substrate charging effects, were studied to distinguish possible origins. The results indicate that the conductivity decrease is not caused by substrate charging, but by irradiation-induced defect formation. [DOI: 10.1380/ejssnt.2011.103]