The rheological characteristics of clay-water systems significantly change with pH and electrolyte on account of changes in the concentration and type of exchangeable cations in double-layers. The viscoplastic character of concentrated natural or pure clay-water mixtures has often been reported in the literature. In steady-state conditions clay-water mixtures are generally shear-thinning yield stress fluids. Since clay-water suspensions contain small particles they can easily be tested with conventional laboratory rheometers. Moreover these clay-water systems exhibit various behaviour types and are thus often used as model fluids. Dilute clay-water suspensions for which hydrodynamic or Brownian effects may play a significant role have rarely been studied. Some clay-water systems are well-known as typical thixotropic materials. Thixotropy was first noticed in tests which involve applying a progressively increasing shear stress up to a certain value then decreasing it to zero.