As carbon nanotubes (CNTs) inevitably contain defects, it is of interest to investigate the possibility of artificially introducing defects to tune their properties. This is especially important for applications in electrochemistry and field emission. We report on the electrochemical behavior, through cyclic voltammetry measurements, of multiwalled CNT samples with different defect densities induced through exposure to hydrogen and argon irradiation. Ar exposure leads to CNT charging and reversible/quasi-reversible electron-transfer kinetics, while hydrogen irradiation yields irreversible kinetics. Correlation with Raman spectroscopy implies the termination of residual dangling bonds in CNTs by hydrogen, causing an elimination of reactive sites. (C) 2008 The Electrochemical Society.