The decomposition of formic acid was examined on the (110)-microfaceted surface of TiO2(100) with temperature-programmed desorption(TPD), static secondary ion mass spectrometry (SSIMS), low-energy electron diffraction (LEED), and work function measurements. No ordered structures of formic acid, or of its main decomposition product formate, were observed. Formic acid decomposed on TiO2(100) to formate and a surface proton at temperatures below 250 K. The main TPD products were water, which desorbed below 450 K, and CO, which desorbed at 555 K. Formaldehyde was also observed as a TPD product at 540 K. CO and formaldehyde resulted from formate decomposition. TPD experiments performed on the O-18-enriched surface show that extensive isotopic oxygen incorporation occurred for the CO, water, and formaldehyde TPD products. The incorporation of lattice O-18 into the water product indicates that the acid proton, deposited during formic acid decomposition, was able to abstract oxygen from the surface. SSIMS measurements indicate that incorporation of lattice O-18 into the formate species did not occur until the onset of formate decomposition, implying that lattice oxygen atoms were involved in the formate decomposition process. Studies with coadsorbed (H2O)-O-18 indicate that water was a mild site blocker of formic acid adsorption and/or decomposition sites and that little or no isotopic oxygen exchange took place between the two adsorbed molecules.