Abstract— The evaluation of the equilibrium constants for charge-transfer complex formation has been of interest for five decades. During this time, absorption spectroscopy using Benesi-Hildebrand, or related methods, has been used to obtain the equilibrium constants. These methods require relatively high concentrations of donor or acceptor to be present in solution when weakly bound complexes are studied, conditions that lead to the formation of higher order complexes and inconsistent determinations of these constants. A new method is presented that allows weakly bound charge-transfer complexes to be studied under low concentration conditions and the equilibrium constants to be determined accurately for the first time. Using this method, the equilibrium constant for the formation of 1, 2, 4, 5-tetracyanobenzene/pentamethylbenzene charge-transfer complex was found to be KCT= 6.8 ± 0.3 M−1 with an extinction coefficient at 400 nm of εCT= 150 ± 30cm−1M−1.