Complexes of perchloric acid with interesting physical properties are investigated in details. Single crystals of 2-hydroxyanilinium perchlorate (abbreviated as 2HAP-ClO4), 3-hydroxyanilinium perchlorate (abbreviated as 3HAP-ClO4), and 4-hydroxyanilinium perchlorate (abbreviated as 4HAP-ClO4) were grown by the slow evaporation technique and vibrational spectral analysis was carried out using Far-IR and Fourier transform IR spectroscopy. The density functional theoretical (DFT) computations were also performed at the B3LYP/6-311++G(d,p) level to derive the equilibrium geometry, vibrational wavenumbers and intensities. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology. (C) 2011 Elsevier B.V. All rights reserved.