The electron storage and discharge properties of single wall carbon nanotubes (SWCNTs) are probed by theirinteraction with radiolytically and photolytically produced methyl viologen (MV+•) radicals. Quantitativeestimation of stored electrons amounts to one electron per ∼100 carbon atoms of SWCNT. These storedelectrons are readily discharged on demand by introducing an electron acceptor such as thionine (E0TH/TH2− =0.064 V vs NHE). On the basis of the charge equilibration experiments, we estimate the apparent Fermi levelof SWCNT as 0.039 V versus NHE.