An equation which expresses the behavior of the frequency spectrum for capillary waves occurring in horizontal gas–liquid flows is derived using a dynamic energy balance. This balance assumes that the energy source for these waves is nonlinear interactions between the dominant gravity–capillary waves and that, in the same manner as for single waves, the dissipation of energy is caused by viscosity. Experimental measurements for both water and a water–glycerin solution with a viscosity of 2 cP in an air flow are described well by the theory over a frequency range of 40–100 Hz.