Five chitosans with different degrees of N-acetylation (DAs), molar masses, and origins were depolymerized by nitrous acid treatment in acidic media, leading to water-soluble 2,5-anhydro-d-mannose chain end oligomers with DPn < 20. The kinetics of the reaction was studied, and the best work conditions were found to be 3 h reaction at 50 °C. It was shown that the DPn of oligomers only depends on the quantity of NaNO2 involved. Molar masses or DAs do not have an impact on the depolymerization process when targeting oligomers with less than 20 units. This depolymerization process also leads to free 2,5-anhydromannofuranose (AMF) that might react with the free amines of obtained oligomers to form imines. This reaction is pH-dependent and in acidic condition leads to the formation of 5-hydromethyl-2-furfural (HMF). At the end, the oligomers were purified by dialysis to get rid of most of the free AMF (<5%) and other residual salts and appeared to have no acute toxicity.