A new concentration and determination method for ppb-levels of hexavalent chromium ion in water, based on the reaction between chromium (VI) and iodide ion to form triiodide ion and the collection of the ion-associate of the triiodide ion with a cation surfactant on an organic solvent-soluble membrane filter, has been described. The color intensity due to the ion-associate in the resulting solution, obtained by dissolving the filter and solute, is directly proportional to the amount of chromium (VI) in the range 0.2 - 1.2 mUg. Detection limits better than 0.2 mUg dm−3as Cr (VI) can been achieved with a 100-fold enrichment factor. The diverse components normally present in water do not interfere. The proposed method has been applied to the analysis of several water samples, such as ground water and waste waters. The recoveries of the chromium (VI) added to the samples are quantitative, and results found are satisfactory.