Nanosystems based on zero-valent selenium and biocompatible polymer stabilizers (polyvinylpyr-rolidone with molecular weight (MW) M-W = (10-55) x 10(3), poly-N, N, N, N-trimethylacryloyloxyethylam-monium methylsulfate with M-W = (30-250) x 10(3) and polyethylene glycol with M-W = (1-40) x 10(3)) are studied by means of static and dynamic light scattering, and the resulting data are compared. Dense spherical multimolecular nanosystems are found to be formed. Morphological and thermodynamic characteristics of selenium-containing nanosystems, depending on the nature and MW of the polymer stabilizer, are determined. It is shown that the properties of nanosystems can be adjusted by varying the molecular weight of the polymer stabilizer.