Mixed-reactant fuel cells are unconventional electrochemical power sources which use a thermodynamically unstable mixture of fuel and oxidant. In the work described here a 2-phase mixture of alkaline potassium formate (liquid)/oxygen(gas) is the fuel/oxidant combination used in the "flow-by" mode with mixed-reactant fuel cells, developed in the laboratory from a 10E-4 m(2) single cell to a bipolar stack of nineteen 35E-4 m(2) cells. With Pd/Ag anode/cathode catalysts, operating at 500kPa(abs), 80 degrees C the single cell with a Pd load of 0.054 kg m(-2) reaches a superficial power density of 4,000 W m(-2) at 10,000 A m(-2). Tests of the 19 cell stack with a Pd load of 0.036 kg m(-2) at 300kPa(abs), 60 degrees C give power output up to 120 W, with a corresponding volumetric power density about 400 kW m(-3). To demonstrate the fuel cell performance three nominally 100 W 19 cell stacks are wired in parallel to a 250 W electric motor to drive a modified electric scooter, with on-board fuel, oxidant and an adult passenger, for about 15 minutes at 10 km h(-1). To our knowledge this is the first time mixed-reactant fuel cells have been used alone to power a vehicle.