The terahertz spectra of all the microspheres showed an increase in absorption with frequency and temperature over the entire investigated range in line with previous measurements of amorphous molecular solids (Figure ). As expected for non-crystalline materials, no discrete spectral features were present and the spectra were dominated by the monotonous increase with frequency that is characteristic for the rising flank of the peak due to the vibrational density of states (VDOS). In contrast, the refractive index subtly decreases with increasing frequency. To further investigate the relationship between the increase of absorption coefficient and temperature we examined the temperature dependent changes in absorption losses at a frequency 1 THz in more detail. Given the lack of distinct spectral features we chose the frequency of 1 THz. The rationale for this choice is based on the fact that the signal-to-noise ratio of the measurement at this frequency is high and that we know from our previous work that at a frequency of 1 THz the minimum in losses in the spectral response is exceeded for all samples studied and hence the absorption is clearly dominated by the VDOS. The changes in absorption at a frequency of 1 THz with temperature for the microsphere samples of PLGA 50:50 and PLGA 75:25, are plotted in Figures and respectively. For all the materials the absorption coefficient was found to increase in a linear fashion with increasing temperature and a number of distinct temperature regions can be identified for each material. T g,β was defined as the intersection point of the two best-fit linear fits at low temperatures for all cases, and T g,α was defined as the intersection point of the two best-fit linear lines at high temperatures, as outlined in. For all of the microsphere samples the change in absorption with temperature can be observed to take place over three distinct regions and two transition temperatures, T gβ 0.5 and T gα , as determined using the methodology outlined above (see Figure , Figure and Table ). In this work we attempt to explain the origin of the transition temperatures by proposing a physical picture of the change in the microsphere dynamics with temperature and relate this to the free volume approach, and to peptide and polymer interactions. It is worth noting that the values of T g,α , as determined from the THz-TDS experiments, are in good agreement with our own calorimetric measurements, T g,DSC , as well as the values reported in the literature for these materials.