Positron emission particle tracking (PEPT) can provide quantitative information on particle motion in a mixer. For the first time, the present study clarifies the flow patterns of particles and granules during the process in which granules are formed in high-shear mixer granulation in three dimensions using PEPT. A number of different regions with size-dependent flows account for the variations in frequency and velocity of collisions between differently sized granules in the kernel when segregation occurs in the granulator. These findings offer a better understanding of the product properties and process attributes of high-shear mixer granulation, which will be of direct benefit as pharmaceutical and other products become more complex. (c) 2010 Elsevier Ltd. All rights reserved.