Physical and chemical properties of clays mostly depends on its mineral and chemical composition, particle size and pH value. The mutual influence of these parameters is complex. Illite is the most abundant clay mineral in Latvia and usually used in building materials and pottery. The viscosity and plasticity of Latvian clays from several deposits were investigated and correlated with mineral composition, particle size and pH value. Fractionated and crude clay samples were used. The plasticity index obtained from Atterberg limits varies between the depth of clay samples, the average values being 23 ± 2. The clay samples were investigated with X-ray diffractometer, laser granulometer and rotational viscometer (0.5-200 rpm). The viscosity of clay-water suspensions most rapidly increases when clay concentration in water is more than 35%. Viscosity of investigated clay suspensions is inversely proportional to applied speed.