As a bio-based counterpart of poly(butylene adipate-co-terephthalate) (PBAT), the wellknown

commercially available biodegradable aliphatic-aromatic copolyester, poly(butylene succinateco-

terephthalate) (PBST) has comparable physical and mechanical properties, but its gas barrier

properties, which are very important for packaging material and mulch film applications, have not yet been reported in literature. In this paper, the O2, CO2 and water vapor barrier properties of PBST vs. PBAT were comparatively studied and reported for the first time. Theoretical calculation of O2 and CO2 permeation coefficients via group contribution method was also conducted. The barrier properties of PBST show clear copolymer composition dependence due to different contribution of BS and BT repeat units and composition-dependent crystallinity. Comparing with PBAT, PBST with close copolymer and three-phase (crystalline, amorphous, rigid amorphous) compositions shows 3.5 times O2 and CO2 and 1.5 times water vapor barrier properties. The slower segment movement and less free volume of PBST, and therefore slower gas diffusion in PBST, accounts for its superior O2 and CO2 barrier, while the better hydrophilicity of PBST counteracts partial contribution of slower segment movement so that the improvement in water vapor barrier is not as high as in O2 and CO2 barrier.