Surface viscoelastic properties were studied in the frequency domain for soluble surface molecular layers spread on pentanoic acid solutions. The ripplon spectroscopy technique was successfully applied to the wide band measurement of the surface viscoelasticity in an extensive frequency range of 10(2)-10(6) Hz. The surface elasticity was observed to increase as the frequency extends over the relaxation frequency which is inversely proportional to the time constant of surface formation. The observed relaxation behavior is well explained by the theory in which the time constant is determined by the diffusion rate of the surfactant molecules in the solution. We also discuss propriety of the thermodynamic expressions for the molecular adsorption to the surface of the solution.