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