The assignment of the splitting of the v(C=O) mode in cyclopentanone is reassessed through an extensive vibrational study, including temperature and solvent variation studies, isotopic substitution, and ab initio calculations of monomer and dimer structures and spectra. The results show that the assignment of the splitting of the v(C=O) mode to Fermi resonance is based on several of erroneous observations, and present strong experimental evidence of molecular association through C-H...O hydrogen bonding.