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, 1 H-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 × 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 80°C. 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 ¬= 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.