In this work we present an investigation of the influence of particle surface characteristics on the electrorheological (ER) behavior of suspensions of either pure or modified hematite (alpha-Fe2O3) particles dispersed in silicone oil. The modification consisted of either dehydration or hydrophobization of the particles before preparing the suspensions. A comparison was performed between the electrorheological responses of suspensions with the same volume fraction of hematite particles having different surface properties. The effects of applied electric field strength on the viscosity, yield stress and dynamic moduli of these suspensions were examined. It was found that the usual positive ER response, that is, enhanced values of the yield stress and elastic modulus induced by the electric field were obtained for hematite and, to a lesser extent, for dried hematite suspensions. In contrast, a "negative ER effect", i.e., the reduction of yield stress and elastic modulus upon application of electric field was observed for hydrophobically modified (oleic acid coated) hematite. This means that the field produces destruction of structures rather than their build up, above a threshold electric field strength. (c) 2012 Elsevier Inc. All rights reserved.