An important challenge in the field of electrorheology is identifying low-viscosity fluids that would exhibit significant changes in viscosity, or a yield stress, upon the application of an external electric field. Our recent research showed that optimal compositions of mixtures, 10 wt % sulfonated polyhedral oligomeric silsesquioxanes (s-POSS) mixed with polydimethyl siloxane (PDMS), exhibited significant electrorheological activity. Here we show that s-POSS/PDMS mixtures containing polystyrene (PS) fillers, of micrometer-sized dimensions, containing as little as ∼1 wt % s-POSS, exhibited an increase in ER activity by an order of magnitude, beyond that of s-POSS/PDMS mixtures. The dynamic yield stress was found to scale with the particle diameter, a, as τy ∝ a0.5 and with the electric field as τy ∝ E1.5–2.5; this behavior is reasonably well understood within the context of dielectric electrorheological theory.Keywords: ; additives; electrorheological fluid; nanocage; polar molecule (PM) ER fluid; polystyrene; silsesquioxane;