A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio єI/єf. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/єf, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of єI/єf. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists between the measured shear yield stress, current density, and the applied breakdown field for oxidized aluminum particles suspended in silicone oil and those predicted by the present model. Recommendations are given for the design of ER fluids with high yield strength and reasonable current density.