Energetics of the boron substitution near the Si(100)-2 X 1 and Si(111)-root 3 X root 3 surfaces have been studied using the semi-empirical cluster AM1 method at substitutional positions at the Si(100)-2 X 1 surface. It has been found that in the both cases boron prefers to be in the second layer, that is it occupies substitutional positions that are bonded with dimers of the Si(100)-2 X 1 surface and S-5 sites under adatoms of the Si(111)-root 3 X root 3 surface. The energies, counting from the fourth bulk-like layer, for the Si(100)-2 X 1 and Si(111)-root 3 X root 3 surfaces are -1.3 and -2.1 eV, respectively. These values may be interpreted as corresponding segregation enthalpies.