Carboxylates adsorbed on solid surfaces are important in many technological applications, ranging from heterogeneous catalysis and surface organo-functionalization to medical implants. We report here the first experimentally determined enthalpy of formation of any surface bound carboxylate on any surface, formate on Pt(111). This was accomplished by studying the dissociative adsorption of formic acid on oxygen-presaturated (O-sat) Pt(111) to make adsorbed monodentate and bidentate formates using single-crystal adsorption calorimetry. The integral heat of molecular adsorption of formic acid on clean Pt(111) at 100 K is 62.5 kJ/mol at 0.25 monolayer (ML). On O-sat Pt(111), the integral heat of the dissociative adsorption of formic acid to make monodentate formate (HCOOmon,ad) plus the water hydroxyl complex ((H2O-OH)(ad)) was found to be 76 kJ/mol at 3/8 ML and 100-150 K. Similarly, its integral heat of dissociative adsorption to make bidentate formate (HCOObi,ad) plus (H2O-OH)(ad) was 106 kJ/mol at 3/8 ML and 150 K. These heats give the standard enthalpies of formation of adsorbed monodentate and bidentate formate on Pt(111) to be -354 +/- 5 and -384 +/- 5 kJ/mol, respectively, and their net bond enthalpies to the Pt(111) surface to be 224 +/- 13 and 254 +/- 13 kJ/mol, respectively. Coverage-dependent enthalpies of formation were used to estimate the enthalpy of the elementary reaction HCOOHad -> HCOObi,(ad) + H-ad to be -4 kJ/mol at zero coverage and +24 kJ/mol at 3/8 ML.