Hydrogen abstraction by gaseous X radicals (X = H, F, CI, Br) from H-terminated 4H-SiC(0001) surfaces, has been investigated theoretically using Car-Parinello-type molecular dynamics. Experimental temperatures of 1873 and 2573 K were applied. For both the Si(0001) and the C(0001) surfaces, the hydrogen abstraction reactions were,found to be exothermic. As a comparison, the adsorption of H to the corresponding surfaces was studied. The H adsorption energy was found to be more exothermic than the abstraction energy for each of the investigated cases. This indicates that it is more favorable for the surface H atoms to remain bonded to the 4H-SiC surfaces than to become abstracted by any of the gaseous radical atoms H, F, Cl or Br.