The stretching vibrations of the –CH 3 and –CH 2 – 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., CH 3 CD 2 OH and CD 3 CH 2 OH, 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 CH 3 -ss and CH 2 -ss modes overlap in the ∼2875cm −1 peak, the CH 3 -as and CH 2 -ss-Fermi modes overlap in the ∼2970cm −1 peak, and the ∼2930cm −1 peak belongs to the CH 3 -ss-Fermi mode, respectively.