The electrorheological behavior of suspensions of two types of inorganic particles, with different size and shape-granular aluminum hydroxide and flaky talc-coated with a polyaniline (PANI) base overlayer, is presented. In suspensions of uncoated particles in silicone oil, the particle size and shape affects the character of the organized structure produced in an electric field and, consequently, also the flow properties. After the particles are coated with PANI base, the electroviscosity for both aluminum hydroxide and talc at high electric-field strengths becomes comparable. This means that the effect of the surface PANI layer has become dominant and the inherent particle nature is less important. The electrorheological efficiency, expressed as relative low-shear electroviscosity of the PANT-coated inorganic particles, is compared with that of PANI particles alone. In spite of the higher electroviscosity of suspensions of PANI particles, their ER efficiency merged with the PANI-coated inorganic powders due to high field-off viscosity of PANT suspensions. (C) 2003 Elsevier Science (USA). All rights reserved.