Activated carbons prepared from firwoods by means of a steam activation method at 900degreesC for 1-7 h are demonstrated as promising materials for supercapacitors. The carbons exhibit high-power, low equivalent series resistance and highly reversible characteristics between -0.1 and 0.9 V in aqueous electrolytes. The pore structure of the carbons is systematically characterized by the t-plot method based on N-2 adsorption isotherms. The adsorption equilibria of tannic acid, methylene blue, 4-chlorophenol and phenol from aqueous solutions on such carbons are perfectly fitted by the Langmuir equation. All the steam-activated carbons prepared at different activation times (t(A)) display ideal capacitive performance in aqueous media. This is attributed mainly to the development of mesopores (with an average pore diameter, D-P, between 2.68 and 3.04 nm), which depends strongly on tA. The average specific capacitance of a steam-activated carbon with a tA of 7 h, as estimated from cyclic voltammetic curves measured at 200 mV s(-1), reaches 120 F g(-1) between -0.1 and 0.9 V in acidic electrolytes. The capacitive characteristics of steam-activated carbons in NaNO3, H2SO4 and HNO3 can be roughly determined by the adsorption data of species with suitable molecular weights. The results indicate that the observed increase in double-layer capacitance arises mainly from the development of mesopores. (C) 2004 Elsevier B.V. All rights reserved.