We reinvestigated the two C=C stretching modes of the five-membered rings of ET (ET = bis(ethylenedithio)tetrathiafulvalene), namely, v(2) (in-phase mode) and v(27) (out-of-phase mode). The frequency of the v(27) mode of ET+ was corrected to be similar to 1400 cm(-1), which was identified from the polarized infrared reflectance spectra of (ET)(ClO4), (ET)((AuBrCl2)-Cl-2) and the deuterium- or C-13-substituted compounds of (ET)(AuBr2Cl2). It was clarified from DFT calculations that the frequency of the v(27) mode of the flat ETO molecule was significantly different from that of the boat-shaped ETO molecule. We obtained the linear relationship between the frequency and the charge on the molecule, rho, for the flat ET molecule, which was shown to be v(27)(rho) = 1398 + 140(1 - rho) cm(-1). The frequency shift due to oxidation is remarkably larger than that reported in previous studies. The fractional charges of several ET salts in a charge-ordered state can be successfully estimated by applying this relationship. Therefore, the v(27) mode is an efficient probe to detect p in the charge-transfer salts of ET. Similarly, a linear relationship for the v(2) mode was obtained as v(2)(rho) = 1447 + 120(1 - rho). This relationship was successfully applied to the charge-poor molecule of theta-type ET salts in the charge-ordered state but could not be applied to the charge-rich molecule. This discrepancy was semiquantitatively explained by the hybridization between the v(2) and v(3) modes.