Sulfenic acid is known to be formed upon the thiol oxidation in the presence of H 2 O 2 . The reduction potentials show that the formation of such chemical entity is a thermodynamically favoured oxidation process and it is herein considered as a potential intermediate in the formation of the NOS bond. The formation of such intermediate is further examined in Figure , where its formation upon the oxidation of the thiol/thiolate with H 2 O 2 is shown. It can be seen that its formation is an exergonic process. The caveat of this intermediate is that its further oxidation leads to the formation of the thermodynamically very stable sulfinic acid with a low kinetic barrier (7.0 kcal/mol), shown in Figure (b), arriving at a reaction dead-end. Therefore, the employed concentration of the oxidant species could be a crucial aspect, otherwise one will overoxidise the thiol/thiolate species leading to sulfinic acid formation.