To further investigate the formation of C−H···O bonded dimers in liquid 4-methoxybenzaldehyde (4MeOB), previously suggested by Raman observations, 1H and 17O NMR chemical shift studies have been carried out for solutions of 4MeOB in CCl4. In addition, Raman and FTIR spectra have been obtained and a set of ab initio calculations has been performed in order to determine possible dimer structures. Considering the 17O NMR spectra, the 15 ppm shielding effect observed for the carbonyl oxygen atom upon concentration increase is strong experimental evidence of hydrogen bonding in liquid 4MeOB. In addition to the previous Raman observations, this result clearly supports the engagement of the carbonyl group as an acceptor in a C−H···O hydrogen bond. The ab initio calculations at the HF/6-31G* and B3LYP/6-31G* levels suggest several possible dimer configurations whose relative energies are within a 5 kJ mol-1 range. All the structures found present two hydrogen bond contacts, and the most stable ones have two carbonyl oxygen donors. The most significant dimer forms in the liquid phase are expected to arise from these configurations.