The time-dependent THz spectra of some chemical materials after being dipped in solvents or after application of spots of solvent were observed. Some specific peaks disappeared, increased in intensity, or newly appeared. These peaks showed different features after spots of different solvents were applied. The evidence suggests that these peaks can be attributed to the vibration of inter-molecules, and shows that the solvents significantly affect inter-molecular vibrations. This method will help in assigning the THz band, and will be useful in researching molecular interactions. (c) 2005 Elsevier B.V. All rights reserved.