Ultrathin oxide layers on hexagonal SiC surfaces were studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). SiC(0001) and SiC(000 (1) over bar). samples were ex situ prepared using thermal hydrogen etching or a microwave powered, hydrogen plasma treatment. A well ordered (root 3X root 3)R30 degrees reconstructed surface is observed by LEED: immediately upon introduction into vacuum. The samples contain oxygen of approximately one layer equivalent bonded to Si atoms as indicated by AES. From a full dynamical LEED structure analysis carried out for the SiC(000 (1) over bar) surface the crystallographic structure is determined: The silicon oxide is arranged as a silicate (Si2O3) layer on top of the SiC substrate, forming rings of (root 3X root 3)R30 degrees periodicity with twofold coordinated oxygen atoms in the topmost position. The oxygen incorporation into the surface presumably proceeds via rapid oxidation in air of the well ordered topmost substrate bilayer. The extreme stability of the resulting surface reconstruction is caused by the absence of dangling bonds in the surface terminating silicate layer. On the SiC(0001) surface a similar silicate type layer terminated by its oxygen atoms is found by the LEED analysis. However, the adlayer and substrate are bonded via a linear Si-O-Si bond.