An extensive experimental study of pure hydrolysis (without oxygen) in polyamide 6 is presented in this paper. 250 micron thick film samples were immersed in pure deoxygenated water at 5 ageing temperatures for almost 2 years. Hydrolysis leads to chain scission, an increase in crystallinity and more surprisingly in a large rise in water content. A new kinetic model considering this water content increase is proposed. The modelled data are confronted with experimental values. It is mandatory to consider this increase in water content in order to be able to predict chain scission at a high level of degradation. It appears that in the absence of oxygen, hydrolysis in polyamide 6 is a slow process when the pH is neutral (7 pH) and obeys an Arrhenius law with an activation energy of 106 kJ/mol. This new model can be used to predict the leaching of macromolecules from the polymer to external water that is of great interest in the framework of ocean pollution by microplastics.