The adsorption and reaction of formic acid on clean and oxygen-modified Mo(110) surfaces have been studied using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The adsorption of formic acid on Mo(110) and oxygen-modified Mo(110) surfaces is largely irreversible. Only similar to 30% of an adsorbed monolayer desorbs molecularly from the dean and oxygen-modified surfaces. However, the distribution of the decomposition products and the reaction pathways are significantly different on the two surfaces. Formyl species, monodentate and bridge-bonded, are found on both surfaces, with the monodentate promoted by the presence of oxygen, apparently due to a site-blocking effect. A CO2delta- species has been identified for the first time as an intermediate in the decomposition of the formate species on the oxygen-modified Mo(110) surface.