We investigate the room temperature interface formation of the Na/carbon-rich beta-SiC(100) surface by core level and valence band photoemission spectroscopy using synchrotron radiation. The deposition of a Na layer at saturation coverage results in surface metallization as evident from the presence of plasmon loss features at core levels, and from a Fermi edge building-up in the valence band. Furthermore, large chemical shifts at both C 1s and Si 2p core level indicate reactive interface formation and surface disruption with significant adsorbate-substrate charge transfer. The Na-Si bond establishment leads to subsequent breaking of the C-Si bonds leaving additional C atoms on the surface as carbon/graphite clusters. Exposure of the Na/beta-SiC(100) interface to a small amount of oxygen results in the removal of surface metallization.