A detailed analysis of the influence of a solvent modelled as a continuum defined by its static and high frequency dielectric constants, ε and ε∞, respectively, on the force constants of a molecule in solution is proposed. It is applied to the computation of the solvent-induced frequency shifts in the vibrational spectrum of formaldehyde. The sign and the order of magnitude of the effect are correctly predicted at the scaled Hartree–Fock, MP2 or DFT levels. The computations performed on conveniently distorted systems allow us to separate the influence of the reaction field on the molecular vibrations from the influence of anharmonicity, by means of the geometry distortion induced by the solvent. The anharmonicity contribution appears to be the major contri bution to the overall effect.