The existence of two mechanisms involved in the ultrasonically induced depolymerization of chitosan is evidenced. The first leads to a rapid scission of polymer chains and a lowering of the polydispersity, and the second is responsible for obtaining short polymer chains and oligomers with a polydispersity increase. A systematic experimental study allowed us to identify and quantify the main parameters influencing the chain scission kinetics. Consequently, using a "master curve" approach, a general law of variation of the molecular weight during the depolymerization is proposed. This law can be used in various experimental conditions to easily control the production of chitosan chains of precise length and low polydispersity or a collection of chito-oligosaccharides (COS). Characterization of the latter by H-1 NMR and MALDI-TOF mass spectrometry shows their high purity and an unchanged primary structure.