In this paper, ibuprofen from a commercial source and its fractioned samples with narrower size distribution were characterized to assess the effect of particle properties on compaction characteristics. The compaction behavior of binary mixtures of ibuprofen with spray-dried lactose was also studied. The tablet in-die densification rate and tablet out-of-die porosity and tensile strength were measured for all samples. It was found that the particle size of ibuprofen does not affect the yield stress as derived from a Heckel plot, however the yield stress increases with the increase of ibuprofen particle size in the binary mixtures. Particle size also affects the tablet out-of-die properties, with tablet porosity and tensile strength increasing with the decrease in particle size. The effect of adding a weak compacting powder such as lactose on the tablet tensile strength is very much dependent on the ibuprofen particle size and mass ratio of the binary mixtures. Mixing lactose with ibuprofen of similar size in equal mass has no effect on the tensile strength of the tablet whereas mixing it with ibuprofen of larger size reduces the strength compared to ibuprofen alone. Adding a smaller amount of lactose can lead to an increase in tablet strength, even though the particle size of ibuprofen and lactose is quite different. Theoretical analysis on the tablet strength based on particle-particle bond strength was also carried out to explain the experimental results. (C) 2012 Elsevier B.V. All rights reserved.