Abstract The reason why HDO prefers to form a deuterium bond instead of a hydrogen bond is discussed. The average structure of a complex may differ for different isotopomers if the intermolecular potential energy function has a flat minimum. This is illustrated for the water complexes of methylsubstituted benzenes. The intermolecular fundamentals of the water ammonia complex are discussed. A possible mechanism for secondary isotope shifts in intramolecular vibrations of complex forming molecules is given.