A solid phase extraction procedure has been established for chromium speciation in natural water samples. The procedure is based on the solid phase extraction of the Cr(VI)-ammonium pyrrolidine dithiocarbamate (APDC) chelate on multiwalled carbon nanotubes (MWNTs). After oxidation of Cr(III) to Cr(VI) by using hydrogen peroxide, the presented method was applied to the determination of the total chromium. The level of Cr(III) is calculated by difference of total chromium and Cr(VI) levels. The procedure was optimized for some analytical parameters including pH, eluent type, flow rates of sample and eluent, matrix effects, etc. The detection limit based on 3 sigma criterion was 0.90 mu g L-1 for Cr(VI). The adsorption capacity of multiwalled carbon nanotubes was 9.50 mg g(-1) Cr (VI). The presented method was applied for the speciation of chromium in natural water sample with satisfactory results (recoveries > 95%, R.S.D.'s < 9%). (C) 2007 Elsevier B.V. All rights reserved.