C-H center dot center dot center dot O hydrogen bonds in liquid 2-cyclohenexen-1-one are studied to assess the vibrational spectroscopic behavior of the C-sp2-H and C-sp3-H donors. The presence of a pseudo-isosbestic point in the nu C&3bond; O region supports the assignment of the two observed bands to two species in equilibrium, considered to be the free and 1:1 associated forms. The values of Delta H degrees=-18.5 +/- 0.6 kJmol(-1) and Delta S degrees=-76 +/- 2 Jk(-1) mol(-1) for the dimerization through C-H center dot center dot center dot O hydrogen bonds were obtained from the dimerization constant at different temperatures. The concentration-dependent intensity of the nu CH2 band profile is ascribed to the presence of a blue-shifted band from the hydrogen-bonded C-sp3-H group. However, the most surprising result is the absence of concentration- or temperature-dependent intensities in the bands assigned to the stretching modes of the C-sp2-H donors.