A coprecipitation procedure has been established for chromium speciation in natural water samples. The procedure is based on the coprecipitation of Cr(III) on thulium hydroxide precipitate. After reduction of Cr(VI) to Cr(III) by using potassium iodide, the presented method was applied to the determination of the total chromium. The level of Cr(VI) is calculated by difference of total chromium and Cr(III) levels. The procedure was optimized for some analytical parameters including pH, sample volume, matrix effects, etc. The detection limits based on 3 sigma criterion were 0.87 mu g L-1 for Cr(III) and 1.18 mu g L-1 for Cr(VI). The procedure presented was validated by the analysis of BCR-144R Sewage Sludge (domestic origin). The presented method was applied for the speciation of chromium in environmental sample with satisfactory results (recoveries > 95%, R.S.D.s < 10%). (C) 2008 Elsevier B.V. All rights reserved.