We studied the dynamics of adsorption/desorption of soluble surfactant (C(12)E(6)) into an insoluble n-dodecyl p-toluenesulfonate (DTS) monolayer at an air/water interface through stress relaxation experiments. The resulting surface tension changes due to a small imposed surface area disturbance, as a function of time, are measured using a capillary wave probe. We derived two theoretical models for this insoluble/soluble mixed surfactant system. The first assumes that adsorption/desorption of surfactant molecules in the presence of an insoluble monolayer is controlled by diffusive matter interchange between bulk and surface, while the second assumes that adsorption/desorption of the soluble component is controlled by an energy barrier mechanism due to the insoluble monolayer. It is found that measured stress relaxation experimental data are in good agreement with the diffusion-controlled model, Both the diffusion characteristic frequency, omega(0), and high-frequency limit surface modulus, epsilon(0), were found to increase with increasing surface concentration of the insoluble monolayer. The increase in omega(0), or the surface relaxation rate, is shown to be related to the static property (partial derivative C-2/partial derivative Gamma(2))(Gamma 1), where Gamma(1), Gamma(2), and C-2 represent the surface concentrations of DTS and C(12)E(6) and the bulk concentration of C(12)E(6), respectively.