The acid-base equilibrium in acid (phenylphosphonic acid, methylsulfonic acid, phosphoric acid, hydrochloric acid, acetic acid, and sulfuric acid)-benzimidazole (BIm) complexes was studied by H-1 NMR spectra, FTIR spectra, and density functional theory (DFT) calculations. The DFT optimized structures of the acid-Blm complexes, the vibrational frequencies of the acidic protons, and potential profiles were applied to study the equilibrium between the acids and Blm. In gas phase, it was shown that the strong sulfuric acid could completely protonate BIm and the other acids could only incompletely protonate Blm. When the polarizable continuum model of DMSO was applied, it was shown that all the acids could completely protonate Blm, except the weak acetic acid and phenylphosphonic acid. Furthermore, the potential profile for the PPoA-Blm shows double-well structure facilitating the free movement of the acidic proton between PPoA and Blm. If an explicit solvent molecule of DMSO was included into an acid-Blm complex, significant changes in equilibrium between acid and Blm are observed. The potential profiles for the MSA-Blm and HCI-Blm show very flat bottom wells, facilitating the free movement of proton between the acid and Blm. These calculations were consistent with the H-1 NMR chemical shifts of the immobile protons of the benzimidazole ring in DMSO-d(6) and the FTIR spectra of the acid-Blm complexes.