The problem of diffraction of an E-polarized plane wave by a perfectly conducting wedge, having the illuminated side loaded with a thin dielectric slab of which the relative permittivity is not much larger than unity, is treated. The solution is given in an integral form, consisting of geometrical optics and associated diffraction terms, by applying an approximate boundary condition at the dielectric surface. The diffraction integrals involved are evaluated asymptotically in terms of Fresnel's integrals. Using the reciprocity principle, the results are extended to the cylindrical-wave excitation. The validity of the results is verified by comparison with experiment for the case of a dipole-fed dielectric-loaded strip.