Seven commercially available pharmaceutical excipients used for direct tableting were studied. Flowability, true and tap density, loss on drying, particle size, specific surface area and scanning electron micrographs were used to characterize these powders with low compaction pressures (lower than 10 MPa). The relaxation mode was analysed to describe the behaviour of each product under strain. Three new models are proposed to aid understanding the physical phenomena involved in the compaction phase, and the Peleg model was applied to the relaxation phase. The models were related with the physical properties measured, and illustrate the sliding friction, the viscoelastic behaviour and the aptitude of particles to fragment during compaction, as well as the elastic behaviour during the stress relaxation of powders.