The surface viscoelasticity of aqueous solutions of surface chemically pure n-decanoic acid has been studied by light scattering from thermally excited capillary waves of frequency between 5 x 10(4) and 9 x 10(5) s(-1). The surface dilatational elastic modulus measured by light scattering, corresponding to such frequencies, agrees with the equilibrium value derived from the pi-ln(c) variation for concentrations below 8 x 10(-6) M and above 6 x 10(-5) M, but systematically exceeds the equilibrium variation between these concentrations. This intermediate concentration range has been described as one of transition between two different states of adsorption (Lunkenheimer and Hirte, J. Phys. Chem. 1992, 96, 8683). The behavior of the elastic modulus can be explained by relaxation involving molecular reorientation within the adsorbed layer, with a relaxation time >> 4 mu s. Such reorientation would be entirely consistent with a change of adsorption state, such as has been suggested for this system. Other aspects of the surface viscoelasticity are briefly discussed.