The vibrational spectrum of ethanol monomer trapped in argon and nitrogen matrices has been recorded in various conditions of temperature and irradiation. The structures and vibrational properties of the anti and gauche conformers have been investigated by ab initio calculations according to the DFT method. The comparison between observed and calculated frequencies allows an explanation of the matrix data and a proposal of a complete assignment for both conformers. The striking spectral changes observed in nitrogen matrices upon temperature cycling in the range 8-30 K are interpreted in term of anti --> gauche conversion. In contrast with these observations, the spectrum obtained in an argon matrix is unsensitive to thermal effects, the anti form being the only one to be stabilized. Monochromatic irradiations in the OH and CO stretching regions were carried out with the purpose of inducing photorotamerization. In all cases only changes of trapping sites were observed, despite an interconversion barrier lower than the energy of the absorbed photons.