A host-guest interaction in a series of Hofmann-type clathrates : M (NH 3) 2Ni (CH) 4·2C 6H 6 where M = Ni, Fe, Co, Zn, Mn and Cd has been studied using IR spectroscopy. The influence of the guest molecules (enclathrate benzene molecules) has been studied through the changes of the shape of the bands due to CH out-of-plane deformations. In some of the clathrates, a considerable splitting of these bands has been observed at low temperature. This is especially true for the band near 980 cm −1. The splitting of this band increases as the volume of the unit cell decreases in the order : Cd, Mn, Zn, Co, Fe, Ni. An attempt has been made to interpret this in terms of tunneling effect, using a simple model based on a double-minimum potential function.