A recent st udy of the range of validity of the surface scattering model by Eftimiu using a two-dimensional surface is re-examined with a more realistic surface correlation function that is everywhere-differentiable and vanishes gradually at infinity. Ii is shown that a sufficient condition on the range of validity is that cos2θ [(kσ)2/(kc)1/2]exp{−[2kc(1−sinθ)]1/2} ≪1where k is the electrical wave-number, 0 is the angle of incidence and a and c are the surface standard deviation and the surface correlation parameter respectively.