Energies, structures, vibrational spectra, and proton transfer in the imidazole-imidazolium complex have been studied by B3LYP/6-311++G** method. This complex is a strongly hydrogen-bonded system with a low energy barrier for the proton-transfer reaction and the rotation of the imidazolium ion along the axis of the complex. The calculations predict that a classical motion of the proton between the two molecules in the complex is allowed. A strong influence of the complex formation on the vibrational spectra has been observed in the N-H stretching mode and in the fingerprint region. Several modes in the fingerprint region have been shown to give suitable indicator bands for complex formation detection in polymer electrolytes where imidazole and imidazolium ions have been added.