Abstract: Ahighly sensitive spectrophotomctric method for determination of silicon,based on the ion-associate formed by molybdosilicatc ( SiMo ) with crystal violet (CV) in the presence of emulsifying agent OP,is described.The molar absorptivity of CV-SiMo was found to be 1.45×10⁵dm³·mol^-1·cm^-1 at 555 nm.Beer's law was obeyed in 0-3.0μgSi/25ml.The proposed method has been applied to the determination of trace amount of silicon in water samples and alloys with Satisfactory results.The solubilization effect of the micelle of the nonionic surfactant on the ion-associate is further discussed.The spectrophotometric properties of the ion-associate was shown to depend mainly upon the properties of the basic dye cations.