The compaction behavior of binary mixtures of lactose and a functional excipient containing 98% of microcrystalline cellulose and 2% of silica was investigated by experiment. The densification process and the role of each component in the compaction behavior were examined by analyzing the effect of the composition on the stress transmission to the powder bed in axial (stress transmission) and in radial (stress transfer) directions. Unlike the behavior of a single component where the applied pressure is predominantly transmitted in the axial direction to the powder bed for densification, the increase of the composition of lactose up to 50% in mixtures increases the stress transfer that becomes greater than the stress transmission. However, for the compaction behavior of mixtures with 50% of lactose, two opposite behaviors were found, below and above a pressure around 70 MPa. Die-wall friction showed also a change at this pressure due to a smoothed contact surface with wall. By increasing the composition of lactose (>50%), the behavior of mixtures becomes dominated by the behavior of lactose where the transmission is better than the transfer. This study demonstrates that the analysis of the compaction behavior of mixtures by recording stress transmission, stress transfer and wall friction give pertinent information about the role of each component in the densification and Could be, to a certain extent, help to the selection of excipient for powder formulation. (C) 2008 Elsevier B.V. All rights reserved.