The oxidation of formic acid on thin, epitaxially-grown Pd overlayers on Au and Pt single crystal electrodes was studied as a function of overlayer thickness ranging from submonolayer coverages up to the equivalent of 10 monolayers. The results were compared with those for massive Pd single crystal surfaces. The electrocatalytic properties of the Pd overlayers were found to depend markedly on their thickness and on their crystallographic orientation. Pd(100) showed the highest catalytic activity of all three low-index faces, and Pd films on Pt(hkl), even when two or three layers thick, generally exhibited a much higher activity than Pd films on Au(hkl) or massive Pd(hkl). In all cases a high resistivity of the Pd surfaces against poisoning by CO was observed, which makes Pd an interesting alternative to the often quickly deactivated Pt as catalyst for organic fuel cell reactions.