A conformational study by vibrational analysis of cyclopentane and halogenocyclopentanes is performed on the basis of the Raman spectra at a very low temperature and the infrared spectra of these molecules trapped in a rigid matrix. The various possible conformations are studied and compared. Only for the form with C s symmetry and axial halogen is there good agreement between calculated and experimental frequencies. Thus the cyclopentyl monohalides are expected to crystallize in this form. A transferable force field is derived for cyclopentane and the cyclopentyl halides, which confirms the hypothesis of a correlation between the vibrational frequencies of these molecules.