The nitrile stretch (ν 2 ) and the combination (ν 3+4 ) modes of acetonitrile (CH 3 CN) were found to be strongly coupled by Fermi-resonance. The frequencies of vibrational modes of acetonitrile in CH 3 CN⋯Li + complex were significantly varied due to a strong ion–molecule interaction. The intensity of the combination mode of CH 3 CN in the FTIR spectrum was substantially enhanced in CH 3 CN⋯Li + complex because the combination band shifted closer to the nitrile stretch band. Here, Fermi-resonances in free CH 3 CN and CH 3 CN⋯Li + complex were studied by ultrafast infrared pump–probe (IR PP) and two-dimensional infrared (2DIR) spectroscopy. Spectral features of Fermi-resonance observed in IR PP and 2DIR signals were presented and the effect of Li + ion on Fermi-resonance were discussed.