Pt/Pd catalysts have recently been found to have exceptional properties in formic acid fuel cells. In this paper, the kinetics of formic acid electro-oxidation on spontaneously-deposited Pt/Pd nanoparticles were measured to provide a basis for fuel cell design. The results confirmed previous findings that palladium covered platinum catalysts showed exceptional activity for formic acid electro-oxidation. The Pt/Pd catalysts were a factor of thirty more active than Pt under the conditions examined. The catalysts also continued to function as CO built up on the catalyst surface and showed reasonable activity. Thus, the catalysts were CO tolerant. In addition, the results showed that the Tafel slope of formic acid electro-oxidation on Pt/Pd was time dependent. Generally, the Tafel slope decreased as COad was deposited on the surface, suggesting a change in rate determining step with CO coverage. The formic acid concentration effect was also studied in detail to give guidance to the practical fuel cell design. Similar studies on Pt nanoparticles were performed for comparison. (C) 2003 The Electrochemical Society.