Matrix pellets containing Eudragit NE 30D were produced by extrusion-spheronization, where the critical control points of the process were detected by means of a factorial design. The factors were the quantity of water, the dosing speed, the spheronization speed and the duration of spheronization. The breaking hardness and aspect ratio were measured and the significant factors were determined. On the basis of the results, a new combination of factors was used, which resulted in matrix pellets with the best characteristics. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.