IR absorption spectra of the three Mannich bases with intramolecular hydrogen bonds were studied in gas phase at T = 380 K and in liquid and supercritical Xe over a wide temperature range (170 - 290 K). The bases were 2-(N,N-dimethylaminomethyl)phenol (I) and two substituted species with 4-Cl (II) and 4,6-di-Cl (III) substituents. The frequencies and relative band intensities were measured in the 4000-100 cm^-1 spectral range. Semiempirical calculations of the frequencies and form of the normal vibrations were performed at the MNDO (PM-3) level. The temperature dependence of the spectral moments of the v_s(OH) band and its transformation upon deuteration were also studied. A model calculation of its shape was performed taking into account a strong anharmonic coupling between the stretching v_s(OH) and low-frequency v_i vibrations of the hydrogen bridge. The effective coupling constant was found to be b̄_i = ᾱ_ssi/v̄_i≈0.7.