The singlet potential energy surface of the O(D-1) + HX --> OH + X, and the O(D-1) + HX --> H + XO reactions, (X = Cl, Br) were explored with the B3LYP and B1LYP hybrid density functional theory methods. When these methods are coupled to the 6-311 ++ G (3df, 3pd) basis set, very reliable information about the relative energies can be obtained (i.e., barrier heights, dissociation energies, etc.). We have examined the above-mentioned reactions, the HOX --> HXO isomerization, and multiple dissociation pathways for both HOX and HXO (X = Cl, Br). The results of our calculations are presented. (C) 2002 Published by Elsevier Science B.V.