The adsorption of a nonionic amphiphile at the solution/air interface has been studied by surface second-harmonic generation. The bulk phase transition of micellization was observed by monitoring the surface-reflected nonlinear signal. The concentration dependence of the second-harmonic signal could be described in terms of a Langmuir isotherm allowing the determination of the free energy of adsorption of the nonionic molecules at the surface. A comparison with experimental data obtained from surface tension measurements suggests a depth-dependent distribution of the surfactant molecules rather than a monolayer-type surface coverage.