Using thermal hydrogen etching or a microwave powered hydrogen plasma for ex-situ preparation, a (root 3x root3)R30 degrees reconstruction can be prepared on both {0001} oriented hexagonal surfaces of 4H and 6H silicon carbide. Their structures are determined using Auger electron spectroscopy (AES) and quantitative low-energy electron diffraction (LEED). The surfaces of both orientations are found to be terminated by a monolayer of silicon oxide, whose bond angles and bond lengths agree with those of bulk SiO2. This adlayer strongly passivates the semiconductor surfaces. Thus, the reconstruction is stable in air and against annealing up to 1000 degreesC in ultra high vacuum. Due to the epitaxial relationship the adlayer may serve as a seed for growing crystalline SiO2 and therefore enable the formation of high quality SiO2/SiC interfaces.