Abstract: The color reactions of quaternary complexes of rare-earth metals with chromazurol S (CAS), 4,7-diphenyl-l,10-phenanthroline and cetyltrimethyl ammonium bromide (CTAB) have been investigated. The absorption maxima of these green complexes appear at 628-642 nm, and their molar absorpti-vities are 1.21×10⁵-1.49×10⁵·mole^-1 · cm^-1. The composition of the green complex of lutetium is Lu³⁺:CAS: 4, 7-dp-phen: CTAB = 1:2:1:4. In the presence of ethyl alcohol(7ml),the absorption maxima of the green complexes show that the bathochromic shift and the molar absorptivities become larger. In this system, CTABexerts either the sensitization effect or the decoloration effect on the color complexes of rare-earth metals at different concentrations. The amount of CTABrequired for complete decoloration of different rare-earth complexes is different. Satisfactory results have been obtained by the application of the quaternary complexes in the spectropho-tometric determination of small amounts of gadolinium in the presence of lanthanum.