The crystal structure of 4-phenyl-benzaldehyde reveals the presence of a dimer linked by the C[double bond, length as m-dash]O and C(9)–H groups of adjacent molecules. In the liquid phase, the presence of C–H⋯O bonded forms is revealed by both vibrational and NMR spectroscopy. A ΔH value of −8.2 ± 0.5 kJ mol−1 for the dimerisation equilibrium is established from the temperature-dependent intensities of the bands assigned to the carbonyl-stretching modes. The NMR data suggest the preferential engagement of the C(2,6)–H and C(10/12)/C(11)–H groups as hydrogen bond donors, instead of the C(9)–H group. While ab initio calculations for the isolated dimers are unable to corroborate these NMR results, the radial distribution functions obtained from molecular dynamics simulations show a preference for C(2,6)–H and C(10/12)/C(11)–H⋯O contacts relative to the C(9)–H⋯O ones.