The effect of intermolecular interactions on the vibrational fine structure of 1s → π excitations in free and clustered carbon monoxide and nitrogen was examined. Small, but distinct spectral changes were observed, including line broadening, changes in intensity and spectral shifts. Using a simple model, these changes were assigned in terms of variations in motion of the core-excited molecule within the molecular clusters. Contributions from intra- and inter-molecular vibrations in core-excited clusters that contribute to the experimentally observed spectral changes, were described.