AbstractComplexes formed between aluminum and cobaltous halides have been studied by means of nuclear magnetic resonance, electron paramagnetic resonance, optical spectra, and x‐ray diffraction. Solutions of aluminum chloride and cobaltous chloride in benzene exhibit two separate Al27 NMR signals: one arising from dissolved AlCl3 and another, shifted by 316 ppm to higher magnetic field, due to Al27 complexed with cobaltous chloride. Shifts and intensity changes in the Al27 resonance occurring upon the addition of LiAlCl4, thiophene, and butadiene to such solutions are described. EPR spectra of the cobalt aluminum complex at 77°K. were obtained and are interpreted by means of the theory of Abragam and Pryce. Optical spectra of cobaltous compounds in the visible and ultraviolet regions are given and related to the coordination symmetry of the compounds. X‐ray diffraction data on the fusion product of AlCl3 and CoCl2 indicate that a compound is formed with the composition Co(AlCl4)2 containing four molecules in a monoclinic cell unit. The results of the various methods are interpreted in terms of a complex of composition AlCl3·CoCl2·AlCl3 which forms charge‐transfer type compounds with butadiene and thiophene. A calculation of the chemical shift of Al27 in the model complex is given which is in good agreement with the experimental shift.