Infrared spectra are reported for the adsorption of formic acid on a polycrystalline silver catalyst after various degrees of oxidation. Three distinct chemisorbed species were identified, two of which corresponded to adsorbed formate on Ag(110) and Ag(111) crystal planes and the other to adsorbed formate on a silver site modified by the presence of subsurface oxygen. Moreover, it was discovered that subsurface oxygen species were primarily located in the vicinity of grain boundary defects. Desorption profiles suggested that coadsorbed oxygen moieties could destabilize chemisorbed formate. Because of the invariance in infrared band positions associated with formate species, regardless of whether or not neighbouring atomic oxygen species were present, a kinetic stabiiization mechanism is proposed. The pivotal role of subsurface oxygen species found in the region of grain boundaries, in the mechanism for the selective oxidation of methanol to formaldehyde, is emphasised.