Abstract I.R. spectra of sixteen halogeno cyclohexanones and α,α′ deutero derivatives have been studied in the 1700–1800 cm −1 frequency range. For some halogenoketones; namely cis 2,6-dichloro cyclohexanone, trans 2-bromo-5-methyl cyclohexanone, 2-bromo cyclohexanone and 2-chloro cyclohexanone, the observed behaviour is not the same for the hydrogenated and deuterated compound. It seems that the ν(CO) region of hydrogenated compounds is “anomalous”. For the two first compounds cited above, a Fermi resonance between fundamental ν(CO) and the fist harmonic of a low frequency band seems a good explanation. In the case of bromo- and chloro cyclohexanone, the explanation is not clear but it remains that a conformational justification alone is insufficient to explain the ν(CO) region of the hydrogenated compounds but sufficient for the deuterated compounds.