A suspension (φ=0.2) of Teflon particles in silicone oil exhibited a negative electrorheological response with a dc electric field. This was in keeping with the fact that the conductivity of the host liquidσfwas considerably greater than that of the particlesσp,giving for the complex dielectric mismatch parameter β*=−1/2. Microscopy observations revealed that with application of a dc electric field the Teflon particles migrated to the positive electrode resulting in a structure which consisted of two layers or zones: (a) a relatively pure liquid and (b) a more concentrated suspension. The thickness of the liquid zone increased with field strength, which led to the decrease in the shear stress with field. A numerical analysis was developed for the electrical properties of the segregated suspension, which gave good agreement with measured values.