The mechanisms of several types of azoxy compound (XN(O)NY) rearrangement reactions have been studied using density functional theory (DFT) with the B3LYP exchange-correlation potential. The substituents X and Y are taken from the set H, CH3,F,C6H5, Cl and CN. The 6-311 + G(d,p) basis set was used to optimize up to ten equilibrium and transition state structures for a given pair of X and Y substituents; except for azoxybenzene where a 6-311(+)G(d) basis set was used. All geometric structures were characterized by a frequency calculation. The reaction path for converting XNα(O)NβY to XNαNβ(O)Y via a concerted intramolecular shift of the oxygen atom from Nαto Nβinvolves three equilibrium and two saddle-point structures. The relatively high (about 70-80 kcalmol1) calculated barrier height to reaction and its independence of the nature of the X and Y substituents is attributed to characteristic orbital, atomic charge and structural factors along the reaction path.Cis↔transisomerization across N=N in the NH(O)NH azoxy compound and across N–N in the ring XNONY oxadiaziridine intermediate is found to have a barrier height of at least about 30 kcalmol-1. Single-point CCSD(T)/DFT energy differences are found to be somewhat smaller than the DFT calculated values.