Single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) were oxidized by NaClO solutions and were employed as sorbents to study sorption characteristics of nickel(II) from aqueous solution. The surface properties of CNTs such as functional groups, total acidic sites and negatively charged carbons were greatly improved after oxidation, which made CNTs become more hydrophilic and resulted in sorption of more Ni2+. The amount of Ni2+ sorbed onto oxidized CNTs increased with a rise in agitation speed, initial Ni2+ concentration and solution pH in the range 1-8, but decreased with a rise in CNT mass and solution ionic strength. The sorption mechanisms are complicated and appear attributable to electrostatic forces and chemical interactions between the Ni2+ and the surface functional groups of the CNTs. The oxidized SWCNTs and MWCNTs have shorter equilibrium time and better Ni2+ sorption performance than the oxidized granular activated carbon, suggesting that both NaClO oxidized CNTs are efficient Ni2+ sorbents and that they possess good potential applications in water treatment. (c) 2006 Society of Chemical Industry