The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinum-based catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4 sites whose turnover frequency for the O2 reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O2 reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN4 sites. These results are interpreted as an increased turnover frequency of FeN4 sites when specific surface N-groups protonate. These unprecedented findings provide a new perspective for stabilizing the most active Fe/N/C catalysts known to date.