We developed a nonperturbative and noncontact method of time-resolved ripplon light scattering for investigating slow dynamics on a liquid surface over a frequency range of 10(-3) Hz to 1 Hz. Surface area of the liquid under study was periodically changed by a movable barrier and the following change in the surface pressure was observed from the ripplon spectrum. This technique was applied for the aqueous solutions of decyl alcohol and a relaxation phenomenon associated with the adsorption and desorption of surfactant molecules was found. The result was successfully interpreted in terms of a relaxation model in which both the potential barrier formed just under the surface and the molecular diffusion in the solution take comparable contributions to the sorption kinetics. The obtained relaxation parameters were discussed in correlation with the Frumkin＇s adsorption model.