Reactions between water clusters containing the superoxide anion, O2˙-(H2O)n (n = 0-4), and formic acid, HCO2H, were studied experimentally in vacuo and modelled using quantum chemical methods. Encounters between microhydrated superoxide and formic acid were found to result in a number of different reactions, including (a) proton transfer, (b) ligand exchange, (c) H2-elimination (affording microhydrated CO4˙-), and (d) dihydrogen transfer (affording H2O2 and microhydrated CO2˙-). The effect of reactant-ion hydration on reaction rates was investigated and the involved reaction mechanisms were elucidated.