International audiencePoly(lactic acid) (PLA) is the most traded biodegradable and biobased material. It is largely used as ecofriendly substitute of conventional plastics. Nevertheless, one of the main limiting factors is its water sensitivity. PLA reacts with water and is hydrolyzed during time, which determines its performance. Limited information related to the hydrolysis mechanism driven by water in vapor state is available in scientific literature. Literature is mainly focused on the effects of water in liquid state. This lack of information is of significant importance, since PLA interacts with water in both phases. This work was aimed to give a full depiction of the chemical and physical changes of PLA in a large range of relative humidity environments (from 50 to 100% RH) and in contact with liquid water. This research clearly showed that the stability of PLA was influenced not only by the chemical potential of water molecules, but also by their physical state due to a different behavior of degradation products. From a practical point of view, the findings of this study can be used as strong scientific basis for giving recommendations about the use of this material in its applications as packaging or mulch films