The interaction between CO molecules and cations in NaY and NaX zeolites has been studied at low temperature and the observed splitting of the CO stretching vibration at low pressure is discussed. At low temperature, it is not possible for the do molecules to penetrate through the 6-ring window to the sodalite cage. Therefore, CO molecules can only interact with cations in the supercage, The shift and the observed splitting of the CO stretching mode in NaY is interpreted as being caused by the distribution of Al-atoms in the 6-ring windows (SU-sites), which leads to the presence of four distinct Sir-sites in NaY. For NaY the splitting of the GO-stretching mode is found to be consistent with the distribution of aluminum and silicon atoms known from Si-29-NMR.