The use of foam to achieve granulation is a new development in wet granulation processes. This paper studies foam granulation by investigating the impact of formulation and granulation process conditions on the granule size distribution in a high shear mixer-granulator. Experimental studies were performed in a high shear mixer-granulator on a lactose and microcrystalline cellulose (PH-101) formulation. The granule size distributions were determined as a function of foam quality (83% or 91%), primary powder particle size (75 pm or 150 pm), HPMC binder concentration (4% or 8%), liquid to solid ratio (20-60%), and impeller rotational speed (295 rpm or 515 rpm). The results showed that decreasing foam quality, or increasing the lactose particle size, liquid binder concentration or liquid to solid ratio increase the average granule size, but the effect is interconnected with impeller speed. Transformation maps were proposed to explain the changes in granule size distribution in response to changes of the studied parameters on the basis of wetting and nucleation mechanisms for foam granulation process. (C) 2011 Elsevier B.V. All rights reserved.