We measure the oxygen-oxygen bond dissociation energy of the peroxyformate anion (HCO3-) using energy-resolved collision-induced dissociation with a guided ion beam tandem mass spectrometer. The analysis of the dissociation process from HCO3- ((1)A') to HCO2- ((1)A(2)) + O(P-3) requires consideration of the singlet-triplet crossing along the reaction path and of the competing OH- and O2H- product channels. The measured oxygen-oxygen bond dissociation energy is D-0(HCO2--O) = 1.30 +/- 0.13 eV (126 +/- 12 kJ/mol). This threshold energy measurement is used in thermochemical cycles to derive the enthalpies of formation for peroxyformic acid, Delta H-f(0)(HCO3H) = -287 +/- 19 kJ/mol, and peroxyformyl radical, Delta H-f(0)(HCO3 center dot) = -98 +/- 12 kJ/mol. These values are in good agreement with computational energies.