Electrorheological (ER) activity of a series of surface-modified polyacrylonitrile-based powders was investigated. The semiconducting powders were obtained by pyrolysis of commercially available polyacrylonitrile fibers in controlled conditions. Some samples were additionally chemically oxidized before thermal oxidation to get materials of different surface characteristics. Selected samples were treated with trimethylchlorosilane in order to modify their surface properties. All materials were carefully characterized by means of elemental analysis, FTIR and XPS (X-ray photoelectron spectroscopy), optical, and scanning electron microscopy. Electrical properties of the powders were also determined. Then ER suspensions were prepared and their ER effect was studied in dynamic conditions. The results were correlated with material properties of the solid phase. It was found that ER effect in the studied suspensions was influenced by ionic polarization processes resulting from the presence of different polar functional groups in the solid material. XPS measurements indicated that materials with high ER effect contained more oxygenate moieties on the grain surface in comparison with materials of lower ER activity. Silane-treated materials exhibited higher ER effect in comparison with their untreated counterparts.