The crystal structure of 2-methoxy-benzaldehyde reveals the presence of several intra- and intermolecular C–H⋯O short contacts, the strongest one forming a dimer linked by the CO and C(3)–H groups of adjacent molecules. In the liquid state, the occurrence of a dimerisation equilibrium is concluded from a splitting in the carbonyl stretching mode. The structure of the possible C–H⋯O interactions has been evaluated by B3LYP/6-31G* ab initio calculations. Analysis of the νC–H region allows the experimental identification of a blue shift of the relevant C–H stretching mode in both the solid and liquid phases.