Barium titanate/insulating oil suspensions were investigated to determine the dielectric polarization mechanisms that govern their electrorheological (ER) behavior. The dynamic yield stress of 19.3 vol % suspensions exhibited Maxwell-Wagner-like frequency dependence, with the dynamic yield stress increasing significantly with ac electric field frequency, as expected for suspensions composed of particles with large dielectric constants. The dynamic yield stress at a given frequency was proportional to E-m, where E is the applied electric field strength and the exponent in increased with frequency. For electric field strengths of 2 kV/mm at 1 kHz, dynamic yield stresses were approximately 500 Pa. Rheological experiments in which particle surface and oil conductivities were varied suggest that the dielectric relaxation of these suspensions is controlled by the particles＇ bulk conductivity. The dielectric relaxation depended strongly on field strength, becoming broader with increasing field strength. Harmonic analysis of the current passing through the suspensions verified that the nonlinear contribution to the apparent suspension conductivity increased with E and decreased with frequency. The current harmonics for barium titanate/dry air suspensions were similar to those of the oil based suspensions, suggesting that nonlinear conduction may arise from field-enhanced dissociation of surface groups, as opposed to field-enhanced dissociation of ion pairs within the continuous phase.