The material-labeling method presented in this paper, coupled with X-ray computed tomography (XRCT), allows in situ non-invasive 3D characterization of certain pharmaceutical processes using relevant materials and process geometry. Microcrystalline cellulose (MCC, Celphere) particles were used in experiments, and lead (II) acetate trihydrate was used to label the MCC particles because it has both high water solubility and high X-ray attenuation coefficient. A 0.5-M lead acetate solution was used to impregnate the Celphere particles, and thus to deliver sufficient contrast in the XRCT shadow images. Selected physical properties of the impregnated particles were examined and found to be similar to the as-received material. The mixture of Pb-impregnated and as-received Celphere particles constituted a model pharmaceutical system to investigate die compaction and blending. Lead impregnation allows particles of low atomic weight to become visually traceable on XRCT by enhancing their X-ray absorption. For the compaction study, the in-die positions of Pb-impregnated particles were monitored as a function of compaction pressure. For the mixing study, a linear correlation between the gray scale of the cross-section images and the percentage of Pb-impregnated particles in the mixture was demonstrated. Two different initial fill configurations were compared: column-filling and layer-filling. Results show that the layer-filled configuration leads to better mixing than the column-filled configuration.(C) 2004 Elsevier B.V. All rights reserved.