Surplus biological sludge from wastewater treatment plants was pyrolysed at 700degreesC in the presence of H2SO4. Sludge-based (SB) activated carbon (AC) was mainly mesoporous in nature, with a surface area of 253 m(2)/g and an average pore diameter of 2.3 mm. Chemviron GW, an AC commercial reference, was mainly microporous with a surface area of 1026 m(2)/g and an average diameter of 1.8 nm. SB AC outperformed the commercial product in the removal of three anionic dyes in solution (CI Acid Brown 283, CI Direct Red 89 and CI Direct Black 168). Chemviron GW performed best for Basic Red 46, which may be related to the relatively small steric size of the dye molecules compared with the size of micropores, and to the greater surface area of the commercial AC. For equilibrium pH values between 5 and 9, the adsorption capacity of SB AC for dyes was significantly modified due to the presence of ionisable surface functional groups while that of Chemviron GW, with a more hydrophobic surface, remained unaltered. (C) 2003 Elsevier Science B.V. All rights reserved.