<p>This work explores feasibilities of forming selenium nanoparticles of different sizes and shapes by means of chemical reduction from a solution of sodium selenite salt of different concentrations in a polymer matrix of purified sodium-carboxymethyl cellulose with the degrees of substitution and polymerization (0.97; 810, accordingly); and determines reaction conditions. It has been experimentally proven that negatively charged carboxyl groups (-COO-) in the macromolecules of sodium-carboxymethyl cellulose interact with selenium ions to form polymer-metal complexes. Homogenous stable selenium nanoparticles with dimensions of 60–90 nm were synthesized in purified sodium-carboxymethyl cellulose solutions, and optimal conditions for synthesis have been determined. Physicochemical research methods have been applied in order to study the properties of solution of sodium-carboxymethyl cellulose containing stable selenium nanoparticles of different sizes and shapes. In particular, the research has made use of physicochemical methods such as IRFure spectroscopy, atomic force microscope (AFM), X-ray diffractometer analysis (XRD) and dynamic light scattering (DLS). The shape and size of the selenium nanoparticles formed in the polymer matrix were studied using the AFM research method. Crystal level of selenium nanoparticles was determined by the X-ray structural analysis method. The chemical bonds formed in the synthesized polymer metal complex [Se(С5 H7 O4 -CH2 OCH2 COO)4 ] 4- n were determined by IR-fure spectroscopy studies. The size of the selenium particles formed in the polymer matrix was determined by the dynamic light scattering method.</p>