Abstract: The organic solvation effects on the colour reactions of Fe(Ⅲ)-chromazurol S(CAS)-cetyltrimethyl ammonium bromide(CTAB) system have been investigated. In the presence of organic solvent the wavelength of maximum absorbance of the multicomponent complex shows slightly hypsochromic shift, and the organic solvent greatly increases the colour reaction sensitivity(molar absorptivities of the ternary complex Fe-CAS-CTABand its solvation complex of acetone are 1.36×10⁵ l·mole^-1·cm^-1 and 2.68×16⁵ l·mole^-1 ·cm^-1 respectively). At the same time the organic solvent could effectively improve the reaction selectivity. The influence of foreign ions has been studied and a new spectrophotometric method for the determination of trace iron recommended. The method could be applied to the direct spectrophotometric determination of microamount of iron in pure rare earths oxide and metallic zinc. The mechanism of the solvation effect has been explored. The composition and the solvation number of the multicomponent complex of iron has also been determined. The effect of organic solvents on the stability constants of the iron complexes with CASand on the acid-dissociation constants (K_a3 & K_a4) of CAS, and on the magnetism of ferric ion has been studied in connection with the role of organic solvents .