The structural and electronic properties of two possible substitutional carbon-doped structures of anatase and rutile TiO2 [i.e., C for O (C@O)- and C for Ti (C@Ti)-doped TiO2] were studied by performing first-principle density functional theory calculations. In C-doped TiO2 with C@O, the band gap changes slightly, but the doped carbon introduces spin-polarized gap states of C 2p orbital in character. Thus, the associated electron excitations among the valence band, the conduction band, and the gap states explain the various visible-light absorption thresholds observed for C anion-doped TiO2. For C-doped anatase and rutile TiO2 with C@Ti, the doped C atom does not induce spin-polarized states. For C-doped anatase TiO2 with C@Ti, the optical absorption energy is reduced by about 0.18 eV, and the C dopant forms a linear O−C−O unit with short C−O distance resembling carbon dioxide, which is consistent with experiment. In C-doped rutile TiO2 with C@Ti, C-doping reduces the optical absorption energy by about 0...