The thermodynamic behavior of chitosan in acidic aqueous solutions with ionic strengths between 0.04 x 10(-2) and 24.30 x 10(-2) M was investigated at 25 degrees C. The intrinsic viscosity and other thermodynamic parameters of chitosan with the molecular weight and the degree of acetylation of 7.14 x 10(5) g mol(-1) and 26%, respectively, were determined and discussed by using both classical equations (Huggins and Fedors) and a new model (Wolf). At low ionic strengths of the solvent, the highest values of the intrinsic viscosity were obtained due to the expansion of the chitosan chains. Two critical concentrations, c* (which separates the dilute-semidilute regimes) and c(+) (at which the dimensions of the polymer coils are considered to have shrunk to their unperturbed dimensions), were estimated, and the effect of the solvent ionic strength on their values was discussed. By increasing of the solvent ionic strength from 0.04 x 10(-2) to 24.30 x 10(-2) M, the persistence length decreased from 17.14 to 4.60 nm, suggesting an increase of the chitosan flexibility due to the decrease of repulsive potential between the polymer chains. The radius of gyration and the persistence length in the unperturbed state were determined as being 52.50 and 3.90 nm, respectively. The viscometric data were corroborated with those obtained by the zeta potential, conductivity, and turbidity measurements for the ionic strength of the solvent between 0.04 x 10(-2) and 42.25 x 10(-2) M.