The effect of mixing particles of different sizes on the electrorheological response of suspensions under steady shear flow was investigated. Two sizes, 15 mum and 50 mum, of monodisperse spherical sulfonated poly (styrene-co-divinylbenzene) particles were used. Several electrorheological fluids were made containing different proportions of small and large particles dispersed in silicone oil, but with constant overall particulate concentration. It was found that the mixed size system produced the highest electrorheological response under the shear rates used (10 s(-1) to 500 s(-1)), which is the opposite trend to previous studies of bimodal systems with larger size ratios.