The vibrational spectra of 4-methoxybenzaldehyde in the liquid phase show pairs of bands whose temperature- and solvent-dependent behaviour can hardly be interpreted only in terms of a cis-trans conformational equilibrium. In particular, the solvent and temperature-dependent intensities of the vC=O band profile strongly suggests the presence of a dimerization equilibrium with a ΔHdim value of ca. 7.6 kJ mol-1. Such dimerization can arise from C-H⋯O interations, as supported by ab initio SCF-MO calculations at the 6-31G* level.