Dielectric response spectra of GaAs surfaces are determined using Kramers-Kronig transformation of the surface photoabsorption (SPA) spectra for the surface conversion caused by Ga-deposition and H adsorption on the As-stabilized (001)-(2 x 4) surface. A method to determine the change in the phase shift associated with the reflectance change is developed to obtain dielectric changes of the surface. The surface dielectric changes show peaks that coincide with the critical points of the bulk GaAs dielectric function as well as features that can be assigned to surface localized states.