Hydrogen peroxide (H 2 O 2 ) synthesis generally requires substantial postreaction purification. Here, we report a direct electrosynthesis strategy that delivers separate hydrogen (H 2 ) and oxygen (O 2 ) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H + and HO 2 – recombine to form pure aqueous H 2 O 2 solutions. By optimizing a functionalized carbon black catalyst for two-electron oxygen reduction, we achieved >90% selectivity for pure H 2 O 2 at current densities up to 200 milliamperes per square centimeter, which represents an H 2 O 2 productivity of 3.4 millimoles per square centimeter per hour (3660 moles per kilogram of catalyst per hour). A wide range of concentrations of pure H 2 O 2 solutions up to 20 weight % could be obtained by tuning the water flow rate through the solid electrolyte, and the catalyst retained activity and selectivity for 100 hours.