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A theoretical study is proposed to reconstruct the gas phase infra-red and Raman spectra of ClH … O(CH3)2 type complexes. More specifically, the effects of electrical and mechanical anharmonicity are considered in detail. Mechanical anharmonicity induces a change in the curvature of the effective potentials governing the hydrogen bond vibration and generates a fine structure of the combination bands. There results a noticeable improvement of the agreement between theory and experiment compared to previously published work, especially on spectral wings; we obtain a better description of the isotopic substitution effect. The effect of electrical anharmonicity, negligible in Raman, is comparatively small in infra-red. Nevertheless, differences between experimental spectra obtained by these two techniques are well understood.