Ab initio and density functional calculations are applied to analyse the structure of the Mannich base ortho-dimethylaminomethylphenol with emphasis on a detailed description of the properties of the intramolecular hydrogen bond. The calculated structures of the components, phenol and dimethylbenzylamine, are compared to the geometries of the hydrogen-bonded and open forms of the Mannich base. Additionally, the gas-phase infrared spectra of the Mannich base and of dimethylbenzylamine are presented. The experimental spectra are confronted with theoretical infrared spectra and interpreted with the aid of a detailed normal coordinate analysis. Trends in the modifications of computed harmonic force constants upon closure of the intramolecular hydrogen bond are monitored and discussed.