In this study, electrorheological (ER) properties of various polyaniline derivatives, namely: poly(o-toluidine) (POT), poly(N-methyl aniline) (PNMAn), poly(N-ethyl aniline) (PNEAn), and poly(2-ethyl aniline) (P2EAn) were investigated. Effects of various parameters such as; particle size, particle conductivity, suspension's sedimentation stability, flow times, concentration, electric field strength, shear rate, frequency and temperature onto ER activity of these polyaniline derivatives/silicone oil (SO) suspensions were investigated. Average particle diameters (d(50)) of four samples were determined by dynamic light scattering as 8.51, 9.08, 11.22 and 13.46 mu m for P2EAn, PNEAn, PNMAn and POT, respectively. It was found that POT has the highest conductivity with a value of 6.87 x 10(-7) S m(-1), and P2EAn has the lowest with 1.01 x 10(-7) S m(-1), among the polyaniline derivatives examined. Fifty four percent of sedimentation ratio was measured after 30 days for all the polyaniline derivatives/SO suspensions studied. The highest electric field viscosity was observed for POT/SO suspensions as eta = 1.04 kPa s. The maximum excess shear stress was obtained for the POT suspensions (25 wt.%) as Delta tau = 917 Pa at E = 2 kV/mm. (c) 2006 Elsevier B.V. All rights reserved.