We investigate the room temperature oxidation of Si-terminated 6H- and 4H -SiC(0001) 3x3 reconstructed surfaces by core level photoemission spectroscopy using synchrotron radiation. On both surfaces, SiO(2) formation for low oxygen exposures suggests very reactive surfaces even at room temperature, Oxygen atoms seem to react below the surface, involving carbon atoms, leading to non abrupt SiO(2)/SiC interface formation. The 4H-SiC(0001) 3x3 surface is less reactive to oxygen exposures than the 6H-SiC(0001) 3x3 one with smaller amount of SiO(2) and a less abrupt SiO(2)/SiC interface formation.