The near infrared combination bands of NH2have been previously shown to respond sensitively to weak complex formation of primary aliphatic amines with various solvents including benzene. The association constant, Kπ, of the 4-phenylbutylamine complex with benzene is evaluated from suitable treatment of near infrared spectrophotometric data from an experimental equilibrium system of the amine and complex. K was found to be 0.10 ± 0.005 1/mole.