We have studied the dynamic properties and relaxations and glass transition behaviour of PLGA and PDLLA by performing variable temperature THz-TDS measurements. We have shown that the T g,α , as measured by THz-TDS, is consistent with the T g results determined by MDSC for each material. A monotonous increase of absorption coefficient with temperature was observed for all the materials examined. Low, medium, and high MW PLGA 50:50 and PLGA 75:25, and medium MW PDLLA exhibit three temperature regimes, with a distinct transition temperatures T g,β and T g,α . We rationalise our experimental results with the concepts of free volume and associate T g,β with the β-relaxation process, where T g,β corresponds to the point at which the material has sufficient amount of thermal energy and free volume for localised motions, here rotations of the C-O bond and methyl side groups, to occur. We conducted DMA measurements in order to further probe the relaxation behaviour of PLGA. We showed that for each of the materials studied, the tan δ versus temperature curve shows two distinct peaks corresponding to the α-relaxation at higher temperatures, and a peak at lower temperatures corresponding to the β-relaxation. Our interpretation of the experimental observations provides a physical explanation to the behaviour of PLGA, based on the PES concept outlined by Goldstein. We attribute the α-relaxation process as the contributing process to T g,α , and define T g,α as the point at which the co-polymer structure has further loosened, and with an increase in free volume the long-range segmental motion