In this study, electrorheological (ER) properties of polythiophene (PT) and poly(oxymethylene)/polythiophene (POM/PT), conducting composites having different compositions were investigated. The particle sizes of the composites were measured by dynamic light scattering method. Conductivities and dielectric properties of these composites were determined. Suspensions of PT and POM/PT composites were prepared in silicone oil (SO) at several concentrations (c = 5-25%, m/m) and their sedimentation stabilities were determined. The flow times of these suspensions at various do electric field strengths were measured. The effects of dispersed particle concentration, particle size, shear rate, electric field strength, frequency, and temperature onto ER activities of these suspensions were investigated. Further, creep tests were applied to the composite suspensions and reversible viscoelastic deformations observed. POLYM. COMPOS., 30:1345-1352, 2009. (C) 2009 Society of Plastics Engineers