In this study, electrorheological (ER) responses of colemanite and polyindene (PIn)/colemanite conducting composite dispersions in silicone oil (SO) were investigated. Antisedimentation ratios of the dispersions were determined. Some parameters which affect the ER properties of the dispersions such as electric field strength (E), shear rate, frequency and temperature were investigated. The rather unusual behavior known as "negative ER effect" was observed for colemanite/SO above E = 1.5 kV/mm and for PIn/colemanite/SO under all the values of electric field strength at volume fraction of 25%. This negative ER response was converted to positive with the addition of non-ionic surfactant.