D-mannitol is a common excipient used in the pharmaceutical formulation of tablets. The glass property of D-mannitol, e.g., the glass transition temperature, Tg, plays a key role in formulation development. Previously, the Tg of D-mannitol could not be measured directly in conventional differential scanning calorimetry (DSC). Recent developments in fast-scan DSC technology have provided new opportunities to characterize the glass transition of D-mannitol. This paper describes use of HyperDSC (R) to characterize the glass transition of D-mannitol. FTIR and Raman spectroscopy are also used to identify different polymorphic forms of D-mannitol.