Viscosity measurements have been carried out on dilute acid aqueous solutions of chitosan and of hydrophobically modified chitosan (HM-chitosan) with three different degrees of C12-aldehyde substitution. The experiments were performed in the presence of different amounts of the cationic surfactant cetyltrimethylammonium bromide (CTAB) (0-30 mm) and with and without salt addition. By using Huggins equation, the intrinsic viscosity [eta], and Huggins constant (k’) were evaluated at different conditions. The intrinsic viscosity was shown to decrease with increasing salt concentration. Addition of CTAB to polymer solutions without NaCl causes the values of the intrinsic viscosity to decline and the values of k’ to rise. These observations are reminiscent of those reported fi om solutions of chitosan and HM-chitosans without surfactant but with increasing salinity. It was observed that [eta] and k’ dependency on the CTAB concentration is also influenced by the degree of hydrophobic substitution, In the presence of 10 mM NaCl, both [eta] and k’ seem to be independent of surfactant concentration for both the unmodified chitosan and the HM-chitosan. The features can be rationalized in terms of screening of electrostatic repulsion.