Despite the fact that amphiphilic block copolymers have been studied in detail by various methods both in common solvents and aqueous dispersions, their hydrodynamic description is still incomplete. In this paper, we present a detailed hydrodynamic study of six commercial diblock copolymers featuring the same hydrophilic block (poly(ethylene glycol), PEG; degree of polymerization is ca. 110 ± 25) and the following hydrophobic blocks: polystyrene, PS 35- b-PEG 115; poly(methyl methacrylate), PMMA 55- b-PEG 95; poly(1,4-butadyene), PBd 90- b-PEG 130; polyethylene PE 40- b-PEG 85; poly(dimethylsiloxane), PDMS 15- b-PEG 115; and poly(ɛ-caprolactone), PCL 45- b-PEG 115. The hydrodynamic properties of block copolymers are investigated in both an organic solvent (tetrahydrofuran) and in water micellar dispersions by the combination of static/dynamic light scattering, viscometry, and analytical ultracentrifugation. All the micellar dispersions demonstrate bimodal particle distributions: small compact (hydrodynamic redii, R h ≤ 17 nm) spherical particles ascribed to "conventional" core-shell polymer micelles and larger particles ascribed to micellar clusters. Hydrodynamic invariants are (2.4 ± 0.4) × 10 -10 g cm 2 s -2 K -1 mol -1/3 for all types of micelles used in the study. For aqueous micellar dispersions, in view of their potential biomedical applications, their critical micelle concentration values and cytotoxicities are also reported. The investigated micelles are stable towards precipitation, possess low critical micelle concentration values (with the exception of PDMS 15- b-PEG 115), and demonstrate low toxicity towards Chinese Hamster Ovarian (CHO-K1) cells.