A novel, physically stabilized hydrogel system composed of chitosan (Chi) or its derivatives [e. g., carboxymethyl chitosan (CMC), sodium carboxymethyl chitosan, or trimethyl carboxymethyl chitosan (TMCMC)] with poly(vinyl pyrrolidone) (PVP) or opened-ring poly (vinyl pyrrolidone) (OR-PVP) were prepared and characterized. TMCMC was synthesized by a novel method with dimethylsulfate as the methylation agent. The synthesized materials were characterized by Fourier transform infrared spectroscopy, (1)H-NMR, (13)C-NMR, and size exclusion chromatography. The mechanical properties, gel fraction, swelling behavior, and water state of the prepared hydrogels were investigated. Gelation occurred when the OR-PVP and Chi solutions were blended within a few seconds. However, the gelation of the OR-PVP and CMC solutions needed pH adjustment. No gelation occurred when the solutions of TMCMC and PVP or OR-PVP were blended. The quaternization or protonization of -NH(2) groups may have prevented the gelation of the solutions. The amino groups of Chi derivatives should have been free to take part in hydrophilic bonds between the two polymers. The physical entanglement of polymeric chains and strong hydrogen bonds between the polymers were considered as mechanisms for the formation of the physical hydrogels. The physical hydrogels showed ionic and pH-sensitive swelling properties. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2761-2771, 2011