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Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force, in sharp contrast to previous observations in the graphite−H₂SO₄ system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an upshift of 320 cm^-1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO₄^-) dopant. This value may prove to be a universal measure of charge transfer in …