Carbon-monolith electrodes for supercapacitors were prepared from GMs (green monoliths) made from pre-carbonized fibers of oil palm EFB (empty fruit bunches) and GMs of pre-carbonized EFB fibers exposed to gamma radiation at 5 kGy, 15 kGy, and 20 kGy. GMs and irradiated GMs were carbonized and activated to prepare ACM (activated-carbon-monolith) electrodes. The gamma radiation affected the pore structure of the ACM electrodes and the electrochemical performance of the supercapacitors; irradiation doses of 0 kGy, 5 kGy, 15 kGy and 20 kGy produced specific capacitances of 121 F g(-1) 196 F g(-1), 11 F g(-1), and 12 F g(-1), respectively. The irradiation dose of 5 kGy appears to be optimum and produces a specific power and specific energy of 236 W kg(-1) and 5.45 W h kg(-1), respectively, representing 34% and 60% increases over ACM electrodes prepared from non-irradiated GMs. (C) 2014 Elsevier Ltd. All rights reserved.