The absorption spectra of four dinitrobenzoic acid isomers were acquired over the range of 0.3~2.2 THz using a terahertz time-domain spectroscopy (THz-TDS) system and over the range of 1 400~1 800 cm-1 using a Fourier transform infrared spectrometer (FTIR). These isomers generate very similar IR spectra but there are substantial differences in the THz region. Vibrational spectra were simulated using density functional theory and the results of these calculations explain the differences and similarities in both the THz and FTIR spectra. THz-TDS technique appears to be a powerful means of identifying isomers in the pharmaceutical and chemical industries.