A homologous series of the chitosan derivative N-[(2-hydroxy-3- trimethylammonium)pro-pyl]chitosan in a 0.33 M CH₃COOH + 0.2 M CH₃COONa mixture is studied via viscometry and static and dynamic light scattering. The molecular masses of the homologous series are determined, and the scaling relationships for the intrinsic viscosity, translational- diffusion coefficient, and hydrodynamic radius of molecules are derived. For the mentioned polymer-solvent system, the mean hydrodynamic invariant is estimated as A ₀ = (3.4 ± 0.1) × 10^-15 kg m ²/(s² deg mol^1/3). The equilibrium thermodynamic rigidity of modified chitosan chains is found to be 20 ± 2 nm. This value, like the parameters of the scaling relationships, suggests that the chitosan derivative under investigation belongs to the class of rigid-chain polymers. It is shown that, in the selected mixed solvent, the molecules of the modified chitosan are more compact than molecules of its nonmodified analog having close degrees of polymerization.