Abstract: This paper studies the chromogenic reaction of Ce⁴⁺ and Arsenazo III to inhibit tin hydrolysis which may interference the determination of cerium in the new tin -cerium or tin -copper alloy plating bath. A new -style of spectrophotometry determination the concentration of Ce⁴⁺ was set up. The result indicates that the maximum absorption wavelength is 538 nm. The relationship between the decrease of absorbance and Ce⁴⁺ content follows Lambert's law when the Ce⁴⁺ content is among 0.4 and 10 μg /mL. The molar absorptivity is 1.2×10⁴ L·mol^-1·cm^-1. The effects of sulfuric acid, glycerin, developer and interfering ions were studied. The determination technology is fast, simple and accurate. When the sample was added standard cerium, the recovery was higher than 98 %. This method can be used in determining cerium amount in the new tin-cerium or tin-copper alloy plating bath.