A novel chemical planarization method was developed for silicon carbide (SiC) and gallium nitride (GaN). This method uses catalytically generated hydroxyl radicals (OH*) to oxidize the wafer surface. OH* are generated by the reductive decomposition of hydrogen peroxide (H2O2) on the surface of the iron reference plate. An extremely flat surface without pits or scratches was obtained. Atomic force microscopy (AFM) revealed that the planarized surface had an atomic step-terrace structure, in which the step height corresponded to a single bilayer of 4H-SiC and GaN. Low electron energy diffraction (LEED) and cathodeluminescence spectroscopy showed that there was no crystallographic damage on the planarized surface.