The global potential energy surfaces of singlet and triplet H 2 O 4 systems have been searched at the B3LYP/6-311G(d, p) level of theory; their relative energies have been calculated at the G2M(CC5)// B3LYP/6-311G (d, p) level. The results show that the most stable intermediate out of the 11 open-chain and cyclic dimers of HO 2 is the singlet HO 4 H chain-structure with C 1 symmetry which lies 19.1 kcal mol -1 below the reactants. The transition states for the production of H 2 O 2 + O 2 (singlet and triplet), H 2 O + O 3 and H 2 + 2O 2 have been calculated at the same level of theory. The results show that the most favored product channel, producing H 2 O 2 + 3 O 2 , occurs by the formation of a triplet six-member-ring intermediate through head-to-tail association with a dual hydrogen-bonding energy of 9.5 kcal mol -1 . The intermediate fragments to give H 2 O 2 + 3 O 2 via a transition state, which lies below the reactants by about 0.5 kcal mol -1 . There are four channels over the singlet surface which can produce 1 O 2 ; all the transition states associated with these channels lie above the reactants by 2.8-5.6 kcal mol -1 at the G2M level. Similarly, the O 3 and H 2 formation channels also occur over the singlet surface with high energy barriers, 5.2 and 74.2 kcal mol -1 , respectively; their formation is kinetically unimportant.