Considerable scientific and industrial interest is currently being focused on a class of materials known as etectrorheological (ER) fluids, which display remarkable rheological behaviour, being able to convert rapidly and repeatedly from a liquid to solid when an electric field (E) is applied or removed. In this article the synthesis, characterization, partial hydrolysis and ER properties of polystyrene-block-polyisoprene copolymer (COP) were investigated. The block copolymer was characterized by GPC, viscosity measurements, H-1-NMR and FTIR spectroscopies, particle size measurements, and elemental analysis. The polystyrene block of the copolymer was partially hydrolyzed by a series of chemical reactions and then converted to a lithium salt by washing with a lithium hydroxide solution. Colloidal suspensions of this polymeric salt were prepared in silicone oil, at a series of concentrations (5-33% m/m). Colloidal stabilities of these suspensions were determined at 20 degreesC and 80 degreesC. Flow times of suspensions were measured under no electric field (E = 0), and under an applied electric field (E not equal 0), and ER Further, the effects of solid particle concentration, shear rate ((gamma) over dot), electric field strength, addition of a polar promoter and high temperature on ER activities of colloidal suspensions were investigated, and excess shear stresses were determined.