While in HClO 4 , this combination of two reaction pathways can continue up to the formation of the *H-adlayer in region IVc, this is not possible in H 2 SO 4 . It is well known from the systems that *HSO 4 inhibits or blocks the ORR. Interestingly, also on surfaces that have a mixed *OH/*HSO 4 adlayer (region III), the ORR is completely inhibited. Apparently, the bulky *HSO 4 species inhibit both the 4electron transfer reaction to H 2 O, via the direct interaction between O 2 and Ru electrode, and also the mechanism discussed above for the 2-electron transfer reaction to H 2 O 2 .