We investigate theoretically the possibility of probing surface magnet
ism by X-ray resonant magnetic scattering combined to X-ray surface di
ffraction. The diffraction spectra for linearly polarized light at res
onance, calculated for the antiferromagnetic UAs(001) and the ferromag
netic Gd(0001) surfaces, are sensitive to orientation and magnitude of
the surface magnetic moments. For Gd(0001) we predict high asymmetry
ratios also for not completely polarized photons.