The reflectivity of periodically corrugated boundaries between non-lossy, isotropic dielectrics with different magnetic permeabilities is studied in a regime where the grating supports only a specularly reflected order. The propagation direction of the incident plane wave may form arbitrary angles with the normal to the mean surface and with the direction of the grating grooves. Relative refractive indices greater and less than unity are considered and particular attention is paid to the cases of index and impedance matching. The changes introduced by the corrugation into the conditions for total reflection and for the existence of Brewster angles are investigated and exemplified numerically by means of a parametric study.