Electrorheological properties of suspensions are considerably affected by hydrophilicity of suspension particles. As a model material, polyaniline base powder protonated with sulfamic, tartaric, or perfluorooctanesulfonic acids provided particles of various hyclrophilicity The experiments revealed that, in the absence of electric field, due to a good compatibility of hydrophobic polyaniline particles with silicone-oil medium, their interactions were limited and the viscosity of suspension was low When the electric field was applied, the rigidity of the polarized chain structure of the particles increased and, consequently, viscosity increased as well In the contrast, the field-off suspension viscosity of highly interacting hydrophilic particles, which are incompatible with the oil, and where particle aggregation may set in, was high especially at low shear rates, and the material had a pseudoplastic character Then, a relative increase in viscosity due to the polarization of the particles or their clusters in the electric field was much lower than in the former case. Due to a different primary structure of suspension, depending on the particle compatibility with the oil the field-off storage modulus of suspensions of hydrophobic particles was lower than the loss modulus, while in suspensions of hydrophilic particles the former modulus dominated In both cases, an increase in elasticity with increasing electric field strength was higher than that in viscosity (C) 2010 Elsevier Inc All rights reserved