The interaction of formic acid with oxidized Pb(100) and Cu(100) single crystal surfaces has been studied by electron spectroscopy (XPS and UPS) and LEED. At 80 K formic acid is adsorbed without decomposition, but on warming to 190 K the reaction M?O + HCOOH ? M(OH)ads(HCOO?) ads occurs. Further exposure of the Pb(OH) (HCOO?) surface to formic acid spectroscopy and LEED. The overall reaction is therefore PbO(s) + 2 HCOOH(g) ? Pb(s) + H2(g) + 2 CO2(g) + H2O(g). The clean Pb(100) surface does not react with formic acid at 298 K. The Cu(OH) (HCOO) surface can not be reduced by formic acid at 298 K, but heating in vacuum to 520 K again regenerates the clean metal surface. When copper has been oxidised beyond the monolayer and exposed to formic acid, only the surface is reduced by subsequent heating in vacuum. Propionic acid adsorbs with decomposition into the propionate ion on both clean copper and oxidized copper at T ? 190 K. Heating the Cu(OH) (CH3 · CH2 · COO?) surface in vacuum at 500 K results in removal of all surface oxygen, although some elemental carbon remains.