Polylactide (PLA) is a bio-based, biodegradable polymer, which is derived from lactic acid and has numerous potential applications, some of which are limited by its moisture barrier and thermal properties. Prior studies have been inconsistent about how moisture sorption in PLA varies with crystallinity and molecular weight. This research is a systematic study of how moisture sorption in PLA depends on molecular weight and aliphatic content via end-group modification. PLA with varying aliphatic content was synthesized by ring-opening polymerization of L-lactide initiated with different long-chain aliphatic alcohols. The terminating end groups were also modified from hydroxyl to acetoxy to eliminate hydrophilic end groups. Molecular weight was controlled by varying the ratio of L-lactide monomer to alcohol initiator, and triethylaluminum was used as the catalyst for precise control over molecular weight. The molecular weight and end-group compositions were verified by gel permeation chromatography and nuclear magnetic resonance. Moisture sorption of the modified PLA samples was measured with a quartz crystal microbalance. Quartz crystal microbalance experiments revealed that both molecular weight and aliphatic content contributed to the sorption properties of PLA. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2543-2549, 2011