Polysorbate 80 has been widely used in the biopharmaceutical industry to reduce agitation-induced aggregation by protecting the protein from interface induced stresses which can be caused upon lyophilisation. The mechanism by which polysorbate 80 acts is still under debate, however, given its widespread use as a stabilising excipient its effects on the dynamics of formulations are of interest to us. We examine the effect of the surfactant polysorbate 80 on BSA in F4 and on the mAb in F7. The THz-TDS data of F4 exhibit a continuous increase in absorption coefficient with temperature, which is indicative of a continuous increase in molecular mobility (Figure ). It is worth highlighting that the values of T g, and T g,↵ are lower in F4 than for all other BSA formulations. It is possible that the presence of the polysorbate 80 molecules results in reduced local barrier heights and hence allow for higher low temperature mobility in F4 compared to the other formulations. Rather than only affecting the local barriers, and with that the value of T g, , the presence of polysorbate 80 also results in increased large-scale mobility, and hence lower values of T g,↵ . This effect is evident in both BSA formulations that contain polysorbate 80 (F3 and F4) and appears to be concentration dependent: the higher the concentration of polysorbate-80 the lower T g,↵ (Table ). Additionally, in contrast to F5 and F6, where the absorption coefficient continuously increases with temperature (Figure ), F7 exhibits confinement of motion at temperatures above T g,↵ , similar to the BSA formulations, F1-3. It is interesting to note that in the presence of polysorbate 80, F7 exhibits higher values of T g, and T g,↵ compared to F5 and F6. This is opposite to what we observed for the BSA formulations. Such a result is in agreement with the hypothesis that in the presence of polysorbate the hydrogen bonded network of the mAb is altered, as mAb1 molecules may bind more strongly to each other than to the interface during lyophilisation . The increase in binding strength between the mAb1 molecules would raise the activation energy barrier for the onset of motions, as indicated by higher values of T g, and T g,↵ . Moreover, the difference in behaviour of F4 compared to F7 could be due to the difference surface characteristics of globular BSA versus the Y-shaped mAb. This could suggest that in the case of BSA, the surfactant molecules interact with the BSA molecules and destabilise the hydrogen bonded network in the solid matrix, however in the case of the mAb the surfactant molecules stabilise it via linking of adjacent hydrophilic and hydrophobic domains within the mAb.