The nonlinear electrorheological （ER） behavior of nonionic surfactant-activated ER suspensions is in vestigated. The influence of three nonionic surfactants (Brij 30, GMO, and GTO) on the electrorheological (ER) response of various alumina/silicone oil suspensions shows similar behavior. The prevalent feature common to all formulations is that the yield stress, Ք₀, initially increases with surfactant concentration, passes through a miximum, then decreases with surfactant concentration. The nonlinear behavior observed at large surfactant concentrations (i.e., τ₀ ＝ Eⁿ, where n<2) arises from field-induced phase separation of a surfactant-rich phase as opposed to field-dependent conductivity of a homogeneous continuous phase.