The stretching vibrations of the -CH3 and -CH2- groups not only overlap but also interfere with each other in the Sum-Frequency Generation (SFG) vibrational spectra of the air/ethanol interface. Using deuterated ethanol samples, i.e., CH3CD2OH and CD3CH2OH, the C-H stretching vibrational modes are clearly assigned for the SFG vibrational spectra of their air/liquid interfaces according to the polarization selection rules. It is found that the CH3-ss and CH2-ss modes overlap in the similar to 2875 cm(-1) peak, the CH3-as and CH2-ss-Fermi modes overlap in the similar to 2970 cm(-1) peak, and the similar to 2930 cm(-1) peak belongs to the CH3-ss-Fermi mode, respectively. (c) 2006 Elsevier B.V. All rights reserved.