Determining the corresponding kinetics and hence the sequence of the elementary steps being followed, however, is not as straightforward. Within the routes that form O* + H 2 O, for example, it is unclear whether a first H + attack is what causes the O-O bond to break (path (i)) or whether it follows as a result of it (path (ii)), or even whether the two elementary steps occur alongside one another in a concerted fashion. The same can be said for the formation of H 2 O 2 with regards to protonation (path (iii)) and desorption (path (iv)).