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We report G-band resonance Raman spectra of single-wall carbon nanotubes (SWNTs) at the single-nanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diameters d t ranging between 0.95 nm and 2.62 nm, we have conclusively determined the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diameter dependent (ω G+= 1591 cm− 1), while the lower-frequency peak is given by ω G−= ω G+− C/d t 2, with C being different for metallic and semiconducting SWNTs (C M> C S). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1< I ω G−/I ω G+< 0.3 for most of the isolated SWNTs (∼ 9 0%). Unusually high or low I ω G−/I ω G+ ratios are observed for a few spectra coming from SWNTs under special resonance …