In this study, the synthesis, characterization, and electrorheological (ER) properties of poly(o-toluidine)/ Zn, (POT/Zn), composites were investigated. Syntheses of the composites were carried out by a chemical method using ammonium persulfate, (NH4)(2)S2O8, (APS), as a free radical initiator. The composites were characterized by Fourier transform infrared (FTIR) spectroscopy, electrical conductivity, magnetic susceptibility, particle size measurements, and scanning electron microscopy (SEM). A series of Zn-containing composites were prepared (0.8-7.6% by mass), and their conductivities were measured to be within the range of 1.7 x 10(-3) -5.0 x 10(-2) SCM-1. Magnetic properties of POT/Zn composites were analyzed by Gouy scale measurements; it was found that their conducting mechanisms are bipolaron. A series of particle size (13, 17, 18, 26, 83 pm) were prepared by ground milling the crude POT/Zn composites. Colloidal suspensions of POT/Zn composites were prepared in silicone oil (SO), at a series of concentrations (10-30%, m/m), and sedimentation stabilities were measured at 25 degrees C. ER measurements showed that the POT/Zn/SO suspension system was ER active. Thus, the effects of solid particle concentration, shear rate, electric field strength, addition of polar promoters, and temperature (25-125 degrees C) onto ER activities of suspensions were investigated. The ER activity of suspensions was increased with increasing particle concentration and electric field strength and decreasing shear rate and showing a non-Newtonian flow behavior. tau = 1.1 kPa shear stress was reached for POT/Zn (4.1 wt %, Zn) composite under E = 2.0 kV, c = 15 (%, m/m), gamma = 1.0 s(-1), and T = 25 degrees C conditions. It was found that the ER activity was slightly decreased with increasing temperature. Further, the addition of polar promoters had no promoting effect on the ER activity of the suspensions and POT/Zn/SO system was classified as dry ER materials. (c) 2006 Wiley Periodicals, Inc.