This article concerns the protonation of chitosan in acetic acid aqueous solution with variable concentration. Comparison with HCl is also drawn. Potentiometry allows the determination of the degree of protonation and the pK(o). Conductimetry and viscometry are combined to follow the role of protonation on solubilization of chitosan. Solubilization occurs for a degree of protonation around 0.5 as previously found for HCl.