Abstract The far infrared spectra of some complexes of pyridines with iodine and bromine have been studied in benzene solution. The infrared intensity of the halogen stretching vibration band has been determined for several complexes with different substituted pyridines. A calculation of transition moments and delocalization moments has been performed and the results are compared with the weight of the dative structure based upon the calculation of the force constants using the frequencies ν X−X and ν N−X. The vibronic contribution to the observed infrared intensity is discussed and a relation between the infrared and the previously determined Raman intensity of ν X−X is examined. A further examination of the difference between iodine and bromine as the acceptor molecule has been included.