The compaction of powdered sodium chloride, sucrose and a coal have been studied by observing changes in specific surface area, pore volume and extent of compaction with pressure. Observations have also been made of appropriate sections using a scanning electron microscope. It has been shown that the initial compaction of the three materials is not governed by the sliding of particles into new equilibrium positions. The compaction of sodium chloride is achieved by plastic deformation of the individual particles, without their fracture. This process continues into the region in which coherent compacts are produced, but work hardening of the material makes it progressively more difficult to compact as higher pressures are reached. For sodium chloride the area of contact, $A$, is related to the compacting pressure, $P$, by the equation ln $P$ = $kA$ + $K$, where $k$ and $K$ are constants. The compaction of both sucrose and coal is dominated by particle fracture. At lower pressures, in particular, the movement of particle fragments into positions of closer proximity is an important secondary mechanism. This movement of fragments into the available pore space is particularly important in the case of sucrose. With coal there is, in addition, a tendency for fragments to remain wedged in the form of chain bridges which maintain a more open structure in the compact. At high pressures, for both materials, plastic deformation of small particle fragments may well be important.