When comparing the behaviour of the different formulations, we found that T g, (F1) > T g, (F2): 262 vs. 219 K. Both F1 and F2 were measured to have similar values of T g,↵ and both exhibit restricted mobility above 340 K. Whilst the difference in relative amounts of histidine and sucrose compared to BSA clearly has a strong effect on the onset of mobility, it does not affect the largescale motions at T g,↵ (336 vs 339 K). In both cases the system exhibited confinement of the protein in matrix at high temperatures, yet, in the absence of histidine and sucrose there was no evidence of any such confinement at temperatures above T g,↵ (Figure ). The difference between the formulations is that in F1 there is less sucrose and histidine available relative to BSA compared to F2. As a result of the lower amount of stabilising excipients, BSA is more likely to aggregate in F1 prior to, and dur- ing, lyophilisation. Increased aggregation results in a significant reduction of local mobility, reflected in a higher T g, , due to the stronger local protein interactions in the absence of stabilising excipient . In the case of more abundantly available sucrose and histidine the individual protein molecules can stabilise their conformations by forming weaker and more flexible hydrogen bonds with the excipients. Given that these protein-excipient interactions must be weaker compared to the protein-protein interactions we consider it likely that the difference in terahertz dynamics which are observed in our experiments are dominated by the protein conformations themselves rather than by the excipients or protein interactions. We further investigated the confinement of BSA in the matrix at high temperatures by subjecting a sample of F2 to a cycle of heating, subsequent rapid cooling followed by a final heating step in order to establish whether we can observe a hysteresis using THz-TDS. Our results show that there is a change in the molecular mobility behaviour of the material during the second heating cycle. The data in Table and Figure , show higher values of m at T T g,↵ in the second heating experiment compared to m ⇡ 0 for T T g,↵ in the first heating cycle. The results would suggest that at T T g,↵ during the first heating cycle, the BSA molecules end up in a confined state that corresponds to a local energy minimum of the PES. However, upon cooling, the conformations appear to change sufficiently that repeated heating from 100 K results in a different trajectory on the PES that does not end up in the confined state at T T g,↵ .