The adsorption and reaction of formic acid, HCOOH, on monolayer and multilayer palladium supported on a Mo(110) surface have been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On multilayer palladium, formic acid adsorbs molecularly at 100 K and desorbs primarily intact upon heating to 200 K. A small amount of the adsorbed formic acid in the first adsorbed layer undergoes decomposition to form CO, H2O, and H-2. No other surface reaction intermediates were identified using HREELS. In contrast to multilayer palladium, monolayer palladium exhibits a higher reactivity toward the decomposition of formic acid to form a surface formate species, as verified with HREELS. However, monolayer palladium shows a relatively low reactivity toward formate decomposition. The formate species is stable on the surface up to 350 K and exhibits an activation energy and preexponential factor for decomposition of 125 kJ/mol and 4.7 x 10(18), respectively. Increasing the Pd thickness to three layers restores the bulk Pd surface chemistry of formic acid.