The population of valence-band electronic states of single-walled carbon nanotubes (SWCNT) was tuned electrochemically in aqueous electrolyte solution. Depending on the applied potential, reversible changes of intensity and frequency of the Raman-active tangential mode and radial breathing mode were observed. The maximum intensity occurs at ca, -0.2 V vs. Ag/AgCl independent of the electrode material used as a support (Pt, Au, Hg). SWCNT are sensitive to photoanodic breakdown upon prolonged exposure to anodic bias and laser light. Reversible bleaching of the first band-gap transition in semiconducting tubes (at 0.7 eV) occurs after anodic polarization.