We investigated vibrational population relaxation for the anti-symmetric stretching mode of the thiocyanate ion (SCN-) in various polar solvents by ultrafast infrared spectroscopy. In aprotic solvents, vibrational population relaxation of SCN- takes place about 2-2.5 times more slowly than in protic solvents. We found that the vibrational relaxation rates and linewidths of the absorption spectra in protic solvents were larger than those in aprotic solvents. We discuss possible mechanisms of vibrational population relaxation and the relationship between relaxation rate and linewidth. (c) 2006 Elsevier B.V. All rights reserved.