The synthesis, characterization, partial hydrolysis, and salt formation of polyisoprene-co-poly(tert-butyl methacrylate) and the electrorheological properties of its suspensions were investigated. The copolymer was characterized by gel permeation chromatography, viscosity measurements, H-1-NMR, Fourier transform infrared spectroscopy, particle size measurements, and elemental analysis. The poly(tert-butyl methacrylate) units of the copolymer were partially hydrolyzed by p-toluene sulfonic acid monohydrate and then converted into a lithium salt. The conductivity of this copolymeric salt was measured to be 1.4 x 10(-9) S cm(-1). Suspensions of the copolymeric salt were prepared in four insulating oils (silicone oil, mineral oil, trioctyl trimellitate, and dioctyl phatalate) in a series of concentrations (5-33%, m/m). The gravitational stabilities of these suspensions were determined at 20 and 80degreesC. The flow times of the suspensions were measured under no electric field (electric field strength = 0) and under an applied electric field (electric field strength not equal 0), and the electrorheological activity was observed. Furthermore, the effects of the solid particle concentration, the shear rate, the electric field strength, a high temperature, and the addition of promoters on the electrorheological activities of the suspensions were investigated, and the excess shear stresses were determined. (C) 2003 Wiley Periodicals, Inc.