Polylactid acid (PLA), as an aliphatic polyester, is one of the most important and studied biodegradable material. The structure, the morphology, the molar mass regulate that biodegrability. During degradation, the process of chain scission occurs through hydrolysis. In this work we have investigated the water penetration through industrial and purified PLA films, in order to understand that degradability. Using different physical measurements, particularly the activation energy of b relaxation, the crystallinity, the water vapour permeability, the glass transition and the ion selective permeability. The complementary of the techniques permits to obtain results that support the physical model presented.