Degradation in quality of fluidisation for Group B powders under vacuum conditions occurs due to an existence of a fluidisation interface that separates the bubbling and static bed near the minimum fluidisation conditions. The significant pressure gradient existing due to bed weight and the particle segregation mainly affects the quality. The aim of the present work is to investigate the relative contribution of the pressure gradient and the presence of the segregation on the quality of fluidisation in vacuum conditions. Further, the effect of morphology of the particles on fluidisation quality is also studied. In addition, fluidisation maps are also obtained that reveal the optimal area of operation for enhanced heat and mass transfer processes. The results indicate that the travel of interface in the bed is affected only when the level of segregation in the bed is high. For intermediate and minimal disparity in size, the quality degradation is caused by the presence of sharp pressure gradient. Changing the morphology of the particle also altered the fluidisation characteristics of the bed and improved the quality significantly. (C) 2013 Elsevier B.V. All rights reserved.