The solvent induced dynamic polarization and the resultant vibrational dephasing have been studied theoretically for acetone in acetonitrile. A molecular dynamics simulation has been used in conjunction with a molecular orbital calculation to obtain the instantaneous C = O stretch frequency that corresponds to the instantaneous polarized structure of the C=O bond. The infrared band shape calculated for the stochastic limit agrees well with the experiment, showing that the solvent induced dynamic polarization is the primary cause of the dephasing of the C = O stretch vibration of acetone in acetonitrile.