Hydrogen abstraction by gaseous X radicals (X=H, F, Cl, Br) from H-terminated 4H-SiC(0001) and (0001) surfaces, has been investigated theoretically using ab initio molecular dynamics. Experimental temperatures of 1873 and 2573 K were applied. For both the Si-terminated (0001) and the C-terminated (0001) surfaces, the abstraction of a hydrogen atom was found to be slightly exothermic or endothermic. As a comparison, the adsorption of H to radical sites on these surfaces was also studied. Hydrogen adsorption was found to be more exothermic than the corresponding abstraction reaction. Assuming an ER abstraction mechanism, the present results suggest that it is energetically more favorable for the surface H atoms to remain bonded to the investigated SiC surfaces than to become abstracted by any of the gaseous radical atoms H, F, Cl or Br. (C) 2003 Elsevier B.V. All rights reserved.