The intracavity dye laser photoacoustic absorption spectra of ethanol, ethanol (1,1-d(2)), and ethanol (2,2,2-d(3)) are reported for the region 16 550-16 700 cm(-1), which contains the O-H fourth overtone (Deltanu(OH)=5) vibration. The distinct absorption bands have been assigned to the trans and gauche conformational isomers. Comparison of the spectra reveals a coupling between the OH and methylene CH vibrations in only the gauche conformer of ethanol, an effect that had not been observed in the fundamental spectrum. Ab initio electronic structure and vibrational frequency calculations are used to clarify and support the analysis of the ethanol OH vibrational spectrum and to evaluate the relative energies of the conformers.