Abstract: In an acidic medium of 1.4 mol·L-1 HCl,and in the presence of nonionic surfactant OP,a stable coordination complex is formed between dibromo-chloro-chlorophosphonazo (CPA-DBC) and La(Ⅲ) having its absorption maxima at 642 nm and a molar absorptivity of (ε642) 1.06×105L·mol-1·cm-1.As compared with the color reaction without the addition of OP,the sensitivity was found to be raised to the extent of 49.3%.Beer′s law was kept in the range of 0-12 μg of La(Ⅲ) per 25 mL of solution.In the application of this method to the determination of lanthanum in molecular sieve samples,an average RSD (n=5) of 2.5% and average recovery of 97.7% were obtained.