A series of complexes between DDT acting as an electron acceptor and certain aromatic electron donors, including benzene, toluene, durene, mesitylene, pentamethylbenzene, and hexamethylbenzene, have been determined in solution using nuclear magnetic resonance spectroscopy. Variations in the proton chemical shifts between complexed and uncomplexed acceptor molecules are found to change relative to one another in the presence of excess donor. The sensitivity to small changes in the electrical environment of the magnetic nucleus of the acceptor molecule in the presence of donor compounds and the study of appropriate nuclear magnetic resonance parameters affords useful information concerning these complexes. Equilibrium constants for Pi-complex formation in CHCl3 and CCl4 have been determined by the Benesi-Hildebrand method or a modification of it.