The vibrational spectrum of 4-methyl-2'-hydroxy-4'-ethyloxyazobenzene (MHEAB) has been studied in low-temperature argon matrices and in CCl4 solutions. Calculations at the DFT (B3-LYP) level of theory with the 6-31++G** basis set, as well as additional IR experiments of deuterated samples and of derivatives without the OH group, have been used to analyze the IR MHEAB spectrum. In the calculations we used the conventional harmonic approach, as well as an approach based on an one-dimensional anharmonic model. It has been shown that introduction of the OH group into the compound and formation of the (OHN)-N-... intra-molecular hydrogen bond cause an increase of the intensity of the stretching N=N mode, but do not affect the frequencies of the azo-bridge vibrations. (C) 2002 Elsevier Science B.V. All rights reserved.