Soybean oil has been hydrogenated electrocatalytically in a solid polymer electrolyte (SPE) reactor, similar to that in H-2/O-2 fuel cells, with water as the anode feed and source of hydrogen. The key component of the reactor was a membrane electrode assembly (MEA), composed of a precious metal-black cathode, a RuO2 powder anode, and a Nafion(R) 117 cation-exchange membrane. The SPE reactor was operated in a batch recycle mode at 60 degrees C and one atmosphere pressure using a commercial-grade soybean oil as the cathode feed. Various factors that might affect the oil hydrogenation current efficiency were investigated, including the type of cathode catalyst, catalyst loading, the cathode catalyst binder loading, current density, and reactant flow rate. The current efficiency ordering of different cathode catalysts was found to be Pd > Pt > Ph > Ru > Ir. Oil hydrogenation current efficiencies with a Pd-black cathode decreased with increasing current density and ranged from about 70% at 0.050 A cm(-2) to 25% at 0.490 A cm(-2). Current pulsing for frequencies in the range of 0.25-60 Hz had no effect on current efficiencies. The optimum cathode catalyst loading for both Pd and Pt was 2.0 mg cm(-2). Soybean oil hydrogenation current efficiencies were unaffected by Nafion(R) and PTFE cathode catalyst binders, as long as the total binder content was less than or equal to 30 wt % (based on the dry catalyst weight). When the oil feed flow rate was increased from 80 to 300 ml min(-1), the oil hydrogenation current efficiency at 0.100 A cm(-2) increased from 60% to 70%. A high (70%) current efficiency was achieved at 80 ml min(-1) by inserting a nickel screen turbulence promoter into the oil stream.