The derivative absorption spectra of the neodymium complex with Semi-Xylenol Orange and cetylpyridinium chloride has been investigated. The characteristic absorption of the complex is approximately 350 times that of neodymium chloride. The fourth-order derivative spectrum has been used to eliminate the interference of the other lanthanides, and to increase the sensitivity by a further factor of 6. Beer's law is obeyed for 0-7.5 microg of Nd in 25 ml of solution. The relative standard deviation for 7 determinations of 1.8 microg/25 ml neodymium was 1.3%. The detection limits were 5.8 ng/ml in the absence of lanthanum and 11.2 ng/ml in the presence of 44 ng/ml lanthanum (or 36 ng/ml yttrium). The method has been used for the determination of neodymium in mixed rare earths, with satisfactory results.