Infrared absorption spectroscopy studies of Si-dimer-terminated 3C-SiC(100) 3x2 and c4x2 surfaces reveal marked differences between their atomic scale reactivity with H, O-2, and H2O and that of Si(100) surfaces. While atomic hydrogen is well known to passivate dangling bonds on all Si surfaces, H exposure on both 3C-SiC(100) 3x2 and c(4x2) surfaces induces a metallic state instead. Furthermore, hydrogenated 3C-SiC(100) 3x2 surfaces exposed to O-2 clearly show that oxygen atoms are inserted below the top surface without any loss in the H coverage at room temperature. Finally, while H2O decomposes into H and OH on both Si(100) 2x1 and 3C-SiC(100) 3x2, subsequent atomic H exposure induces a metallic state on the latter only, creating a thin oxide strip on the top surface separated by a metallic trough. (C) 2003 American Institute of Physics.