Thin bio-based polymer films have been shown to have potential uses in a variety of applications, such as medical coatings, food packaging, and textile fibers. In order to optimize their use, it is essential to understand how these films interact with their environment and how these interactions effect changes in the properties of these films. This paper presents experimental measurements of moisture transport in polylactide (PLA), obtained using a combined Quartz Crystal Microbalance and Heat Conduction Calorimeter (QCM/HCC) and Dynamic Vapor Sorption (DVS). Measurements of moisture sorption in PLA, a moderately hydrophobic aliphatic polyester, versus relative humidity and temperature show that moisture sorption increases with degradation but is only weakly dependent on crystallinity. These results contradict the common assumption that water is exclused from crystalline domains. The composition of the end groups in PLA were modified and no significant end group effect was observed.