Abstract: The paper presents the experimentally obtained optimal conditions for the reaction of immobilization and complexation of Bi (V) with reagents TN-15 (2-oxo-5,6- dimethylthieno [2,3] -pyrimidin-4-one) and TN-249 ( 2-N-5-methyl-6-ethoxycarbonylthieno [2.3-d] pyrimidin-4-one). The dependence of the analytical signal on the nature, concentration and pH of the medium of the reacting components was established, the composition of the complex (Me: R), the molar coefficient of light absorption of the complex (MeR2) and the reagent (HR) were determined, the dissociation constant of the reagent and other physicochemical characteristics were determined. The possibility of the sorption-spectroscopic determination of Bi (V) was shown and the conditions for the selective sorption-spectroscopic determination of Bi (V) in the presence of foreign ions were optimized.