The atomic scale passivation (oxidation, hydrogenation) of the 6H-SiC(0001) 3x3 and 3C-SiC(100) surfaces is investigated by atom-resolved scanning tunneling microscopy (STM), photoemission (core level, valence band) and infrared absorption (ERAS) spectroscopies. Oxygen atom interaction with 6H-SiC(0001) 3x3 takes place well below the surface away from the Si dangling bonds, near the C plane, in Si-O-Si bridge bonding positions. While the 3C-SiC(100) 3x2 surface remains totally inert to H-2, the c(4x2) is highly reactive to H-2 with sticking probabilities approximate to 10(+8) higher than for Si(100)2x1 and molecular dissociation. The H atoms adorb on the Si dangling bond sites. At saturation, H induces a 2x1 surface transformation leading to a H-passivated surface.