Electrorheological (ER) properties of polyaniline suspensions in silicone oil with various experimental conditions were investigated. These suspensions exhibit dramatic changes in their rheological properties which are caused by the formation of particle chain structures induced by an applied electric field. Polyaniline was synthesized by the chemical oxidation of aniline with ammonium peroxysulfate. This study examined the effects of electric field, volume fraction, polymerization temperature and operation temperature on the ER properties of polyaniline suspensions in silicone oil. Rheological measurements were carried out by using a rotational rheometer with a high voltage generator in both controlled shear stress and shear rate modes. Experimental results showed that the ER properties were enhanced by increasing the volume fraction and electric field and were independent of the operating tempertures. The best ER properties were also obtained by using polyaniline particles synthesized at -10 degrees C, and these had a conductivity of similar to 10(-10) S/cm.