It is shown, for the first time, that mixtures of sulfonated polyhedral silsesquioxane cage structures (sPOSS) and poly(dimethyl siloxane) (PDMS) with silicone oil exhibit significant electrorheological (ER) activity. At low sPOSS concentrations, less than 10 wt %, the viscosity is enhanced by approximately 100, which is comparable to the viscosity enhancements exhibited by conventional ER fluids, under the influence of comparable applied electric fields, E = 2 kV/mm. Measurements of the shear stress, sigma, dependencies on E, the conductivities, and relative permittivities reveal that the properties of these POSS/PDMS systems cannot be reconciled with theory developed to explain the behavior of conventional ER fluids.