This paper aims at giving a better understanding of the reaction mechanisms involved in the heterogeneous deacetylation of beta-chitin in relation with the influence of soda concentration (30-55% (w/v)) and the type of sodium hydroxide hydrates formed in solution. The role of temperature (35-110 degrees C) and of the amount of sodium acetate generated in the reaction medium was also investigated. We demonstrated that the type of soda hydrate formed before deacetylation starts and its relative abundance drive the reaction efficiency. Thus, in the first part of this work, we evidenced that activation energies and the global reaction order associated to sodium hydroxide varied as a function of soda concentration. Therefore, we revealed that deacetylation efficiency was emphasized when the less hydrated soda was used, whereas anhydrous soda showed no or very low activity. We also pointed out that various parameters could be responsible for the progressive dehydration of the reaction medium, responsible for the transformation of the most reactive hydrates into less effective species. We underlined that this progressive dehydration could be caused by either one or all of the three following phenomena: alkaline hydrolysis of the polymer, the delivery of sodium acetate in the medium, and the evaporation of water when we process deacetylation at high temperatures and in open reactors. Beside kinetics reasons, we revealed that the transformation of soda hydrates as the deacetylation proceeded was also ascribable for the low reaction efficiency at long reaction times. Thanks to our investigations, we concluded that the amount of water present in the system chitin/soda/water/sodium acetate was the angle stone of complex equilibriums governing the reaction, and we propose soda mono- and dihydrates to be the most active reactants for the chitin deacetylation.