Abstract A new spectrophotometric determination method of hexavalent chromium in waste water and plating baths is described based on the oxidation of beryllon III by chromium(VI) in 0.02 M sulphuric acid medium. The decrease in the absorbance of beryllon III was measured at 482 nm with an apparent molar absorptivity of 5.15 × 10 41·mole −1·cm −1. Beer's law was obeyed for chromium(VI) over the range 0–25 μg/25 ml. After the oxidation of Cr(III) to Cr(VI) by ammonium persulphate, total chromium can be determined. Therefore, chromium(III) can be calculated by subtracting chromium(VI) from total chromium. The detection limit is 0.015 and 0.020 μg/25 ml for chromium(VI) and total chromium, respectively. A sensitive spectrophotometric method for trace Cr(III) and Cr(VI) in waste water and plating baths was developed with good precision and accuracy. The reaction is also discussed.