The extrusion/spheronisaton technique has made a notable contribution to the existing range of pharmaceutical forms, especially in the area of modified-release products. The present work helps to establish guidelines for a rationalised approach to engineering medicinal pellets. This study proposes an approach to monitoring the hydro-textural and morpho-granular characteristics of the product up to its final form. The hydrotextural states, produced by the different unit operations, resulting from the joint influence of the properties of the material and the action of the process, are located on a porosity/water content diagram. The spheronisation stage is studied more closely through the process parameters of spheronisation speed, load and time. What emerges from this study is that the spheronisation stage conserves the saturated state of the extrudates, its role being restricted to that of a shaping process. Moreover, a solid/liquid ratio is established independently of the three operating variables leading to constant particle size distribution. These results orient research toward finding optimum quality by working on formulation/process adjustment. (c) 2005 Elsevier B.V. All rights reserved.