Abstract Raman and i.r. band widths of all the vibrational bands of acetonitrile, CH 3CN and CD 3CN, have been measured in the pure liquid and in solutions of isotopic compounds. Widths arising from the vibrational relaxation, reorientational relaxation and resonance energy transfer have been separated on the basis of the current theories on vibrational band widths. The vibrational relaxation widths estimated for i.r. a 1 bands agree well with the observed isotropic Raman widths. Acceptable values of the rotational diffusion constants have been obtained from the band width data.