Results of X-ray diffraction, IR spectra of the solute either in an Ar matrix or in benzene and carbon tetrachloride solutions as well as DFT theoretical studies are used to discuss the structure and dynamics of 4-chloro-2'-hydroxy-4'-ethoxyazobenzene with particular attention to the OH (.) (.) (.) N intramolecular hydrogen bonding. The packing in the crystalline lattice suggests preferences for the formation of sandwich type dimers in stacks with OH (.) (.) (.) N bridges in two opposite directions. The analysis of the IR spectra supported by DFT calculations was particularly devoted to the manifestations of hydrogen bonding in the nu(OH) and gamma(OH) vibrations. It is demonstrated that there is a substantial contribution of the gamma(OH) overtone to the intense complex absorption in the spectral region around 1600 cm(-1). The mechanical and electrical anharmonicities of the OH group are discussed based on the H/D frequency and intensity isotope effects. The extremely small intensity of the v(OH) band (41 km mol(-1)) is also particular in the present case. (C) 2002 Elsevier Science B.V. All rights reserved.