Spray drying has a wide variety of applications from production of dried milk to pharmaceuticals. The detailed characterization of the drying of the resulting drops is an important component of designing any spray drying process. This study focuses on the drying of hypromellose acetate succinate (HPMCAS) primarily because it has found wide application in pharmaceutical production. Numerous models have been published on the rate of drying and the resulting morphology of spray-dried drops. The drying rate is abruptly changed when the crust or skin forms on the drop. The skin formation in many systems is directly related to the onset of the glass transition state. This paper treats in detail the modeling of the drying of an HPMCAS drop taking into account the radical change in the mutual diffusion coefficient as the concentration changes. The onset of the glass transition corresponds to the formation of the polymer-rich crust at the surface. Experimental data from pendent drop and acoustical levitation methods are compared with the model predictions of the timing of the formation of the crusts. The experiments examine cases with different solvents, multiple airflow rates, multiple gas phase temperatures, and multiple solvent starting concentrations. In all cases the model predictions are in good agreement with the experimental data indicating the importance of considering the diffusion behavior inherent in the drying of polymeric drops.