This paper reports work on the scale-up of a perforated bipole trickle-bed electrochemical reactor for the electro-synthesis of alkaline peroxide. The reactor uses a relatively simple cell configuration in which a single electrolyte flows with oxygen gas in a flow-by graphite felt cathode, sandwiched between a microporous polyolefin diaphragm and a nickel mesh/perforated Grafoil anode/bipole. Both one and two-cell reactors are scaled-up from cathode dimensions 120 mm high by 25 mm wide and 3.2 mm thick (reactor-A) to 630 mm high by 40 mm wide and 3.2 mm thick (reactor-B). The scale-up is achieved by the use of constrictions that prevent segregation of the 2-phase flow in the larger cell, combined with switching from a polypropylene to a polyethylene diaphragm with improved transport properties and raising the electrolyte feed concentration from 1 to 2 M NaOH. For the one-cell reactor-B with a polypropylene diaphragm, operating on a feed of 1 M NaOH and oxygen at 900 kPa(abs)/20 degrees C, the peroxide current efficiency at a superficial current density of 5 kA m(-2) increases from 27% (un-constricted cathode) to 57% with a constricted cathode. The corresponding current efficiencies at 3 - 5 kAm(-2) for reactor-A and the constricted reactor-B are respectively 69 - 64% and 66 - 57%. Under similar conditions at 3 - 5 kA m(-2) the one-cell constricted reactor-B with a polyethylene diaphragm gives current efficiencies of 88 - 64%, and changing to an electrolyte of 2 M NaOH raises this range to 90 - 80%. At 3 - 5 kA m(-2) the equivalent two-cell (bipolar) constricted reactor-B shows current efficiencies of 82 - 74% and at 5 kA m(-2) obtains 0.6 M peroxide in 2 M NaOH with specific energy 6.5 kWh per kg H2O2.