Organic small molecules continue to gain attention for application in light-emitting devices in displays and solid-state lighting. The purification of these materials by sublimation represents a critical obstacle for their high throughput processing. In this work, we find that the purification of the archetypical hole-transporting material N,N '-bis(naphthalen-1-yl)-N,N '-bis(phenyl)-benzidine (alpha-NPD) is controlled by a combination of viscous flow, Knudsen diffusion, and physical vapor deposition. In contrast with other commercially practiced sublimations, steps like diffusion within the solid feed, desorption from the feed particle surface, and mass transfer within the bed of feed particles, do not significantly affect the sublimation rate. This work provides guidelines for the large-scale purification of organic semiconductor materials, and possibly for a broader range of high value small molecule specialty materials. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 1347-1354, 2014