A controlled interfacial synthesis using a permeable membrane has been used to obtain ε-MnO2 of a globular morphology, with a mechanism of formation inferred from SEM results. Specific surface areas of 50-160 m 2 g-1 have been obtained through simple variation of the permeable membrane used in synthesis. This has been shown to have a marked effect on both particle size and pore size distributions. After assembly in supercapacitor test cells with aqueous neutral electrolyte, specific capacitances have been determined through galvanostatic cycling giving initial values as high as 240 F g-1, which drop to 160 F g-1 after 200 cycles at low discharge rates. Excellent cycling behavior is observed at higher discharge rates. © 2010 The Royal Society of Chemistry.