Diffusion is the net action of matter (particles or molecules) and is driven to minimize a concentration gradient. Diffusion in pressure solution refers to the spontaneous process where dissolved minerals move from the place the dissolution occurs, with high concentration and high pressure, to places where concentration is low and pressure is low.

Diffusion is affected by temperature and the concentration gradient. it is generally agreed that to produce a reasonable geological strain rate, the diffusion step of pressure solution is aided by the presence of water (Fletcher and Hofmann, 1974; Rutter, 1983; Green, 1984). The transport ability or the thickness of this water film is affected by pressure and the surface to which adsorption occurs. Please see the link titled 'Factors Affecting Pressure Solution' for more discussion.