Adsorption isotherms of Ni(II) ions from aqueous solutions of nickel nitrate in the concentration range 50-1000 mg/L on two samples of granulated and two samples of fibrous activated carbons associated with varying amounts of carbon-oxygen surface groups have been determined. The adsorption isotherms are Langmuir in shape showing an initial rapid adsorption tending to be constant at higher concentrations. The adsorption of Ni(II) ions increases on oxidation, and decreases on degassing of the carbons surface. The increase in adsorption on oxidation depends upon the nature of the oxidative treatment while the decrease on degassing depends upon the temperature of degassing. The adsorption is related directly to the amount of acidic surface oxygen groups present on the carbon surface.