ABSTRACTSalt weathering is now regarded as an important agent of rock disaggregation in deserts. This paper assesses the relative importance of two potential sources of stress associated with salt weathering, namely crystallisation pressure and volumetric expansion upon hydration, by comparing their ability to cause particle size changes in prepared aggregates of a shale. Two sets of replicate samples in the 2 to 16 mm size range were subjected to 200 24 hour cycles involving respectively conditions favourable to salt crystallisation alone, and to crystallisation followed by salt hydration. An additional set was exposed to 200 cycles of low and high relative humidity respectively but without salt. Crystallisation and salt hydration operating together were no more effective in the production of fines (material>2 mm) than crystallisation acting alone. However, a comparison of the pre- and post-test grain size distributions showed a greater internal transfer among existing grades for the crystallisation-hydration combination. Here hydration augmented crystallisation following the penetration of salt solutions along bedding planes. However, the overall contribution of hydration to salt weathering appears to be minor.