The porous structure and electrochemical double layer capacitance of porous carbons prepared from rice husks by using alkali hydroxide as activating agents were investigated. Three samples of carbons prepared by NaOH-activation, three samples prepared by KOH-activation and two samples of commercial carbons have been studied. The porosity of the carbons was characterized by nitrogen adsorption isotherms at 77 K and electrochemical constant current cycling method was used to measure the double layer capacitance. The specific capacitance of the carbons is not linearly proportional to the surface area. Additionally, the double layer capacitance strongly depends on the pore structure and the functional groups. A specific capacitance larger than 200 F g(-1) was achieved by using the porous carbon prepared with NaOH (activation temperature: 750 degreesC; activation time: 30 min). All the carbons prepared with rice husk in this study have larger double layer capacitance (125-210 F g(-1)) than the commercial grade carbons (78-100 F g(-1)). (C) 2003 Elsevier Science B.V. All rights reserved.