Different classes of excipients with potential for use in the design of novel pelletized formulations manufactured by extrusion-spheronization were examined using factorial experiments. Among the various silicates examined in a model mix with microcrystalline cellulose and lactose wetted with water, kaolin, talc and Veegum F provided improved plasticity for the formation of spherical pellets. Weak bases such sodium bicarbonate and weak acids such as fumaric acid also aided spheronization. Whereas waxy materials such as hydrogenated castor oil and Precirol ATO5, and wetting agent such as sodium lauryl sulphate improved sphericity, these excipients reduced pellet yield by favouring agglomeration. Other materials promoting unwanted formation of over-size pellets were bentonite, citric acid and tartaric acid. The inclusion of Bentone 27, various hydroxide and carbonate bases, and fumaric acid favoured fines production. Collectively the results showed that within classes of excipients, it was not possible to predict the effect of different materials on pellet yield and sphericity. However, Carr＇s index and Hausner ratio calculated from density determinations correlated well with sphericity measured by image analysis.