Light scattering from thermally excited capillary waves has been used to study normal liquid alkanes (15 to 18 carbon atoms in length) as a function of temperature. In all cases the frequency and damping of the surface waves exhibit well-defined discontinuities at a temperature near, but distinct from the melting point. Above that temperature the dat behave as expected for a clean liquid surface; below it they are consistent with predictions for fluid having a structured surface layer through which the surface excess viscoelastic modulus is negative in magnitude. The phenomena suggest the occurrence of a surface-induced phase transition, in which the conformational degrees of freedom of molecules close to the surface are inhibited.