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 vC-H region allows the experimental identification of a blue shift of the relevant C-H stretching mode in both the solid and liquid phases. (C) 2002 Published by Elsevier Science B,V.