Electrorheological (ER) properties of polyaniline (PAni), pumice and polyaniline/pumice composites ( PAPC) were investigated. Polyaniline and PAni/pumice composite were prepared by oxidative polymerization. PAni/pumice particles-based ER suspensions were prepared in silicone oil (SO), and their ER behavior was investigated as a function of shear rate, electric field strength, concentration and temperature. Sedimentation stabilities of suspensions were determined. It has been found that ER activity of all the suspensions increases with increasing electric field strength, concentration and decreasing shear rate. It has shown that the suspensions have a typical shear thinning non-Newtonian viscoelastic behavior. Yield stress of composite suspensions increased linearly with increasing applied electric field strength and with concentrations of the particles. The effect of high temperature on ER activity of pumice/silicone oil systems was also investigated.