The abstraction of chemisorbed hydrogen on the Si(100) surface by atomic deuterium has been studied by direct observation of the reaction products HD. The angular distribution of the HD yield is found to be shifted towards the specular direction of the incident D atoms by about 20 degrees with respect to the surface normal. The strong forward HD desorption is compared with the thermally associative desorption of D-2 peaking towards the surface normal. A direct Eley-Rideal mechanism is suggested for the D abstraction reaction of the chemisorbed H atoms on the Si(100) surfaces.