Monolayers of dodecylammonium chloride formed at the solution-air interface at different electrolyte concentrations have been investigated by dynamic surface light scattering. Pronounced maxima in the damping of capillary waves were observed at surfactant concentrations around 0.1 X CMC. These maxima occur at concentrations just above the break points in the surface tension vs concentration curves caused by first-order phase transitions from a gaseous to a liquid-expanded state. The experimental results were analyzed by comparisons with theoretical calculations based on the standard theory of light scattering due to capillary waves on liquid surfaces, and the molecular theory of the surface tension of ionic surfactant solutions presented earlier by Eriksson and Ljunggren. In this way it was possible to account satisfactorily for the observed concentration and salt dependencies of the central frequency and width of the spectral peak.