—We have synthesized needle- and platelike α-Bi 2 O 3 and Bi 2 O 2 CO 3 nanoparticles. The bulk density of the α-Bi 2 O 3 and Bi 2 O 2 CO 3 powders is 8.5 and 6.9 g/cm 3 . The dielectric permittivity of α-Bi 2 O 3 and Bi 2 O 2 CO 3 has been calculated using dielectric permittivity and dielectric loss tangent dispersion measurements in the frequency range from 2.5 × 10 1 to 1 × 10 6 Hz for 40% suspensions (0.05 and 0.08 volume fractions of α-Bi 2 O 3 and Bi 2 O 2 CO 3 , respectively) in PMS-300 polydimethylsiloxane. The value of ε ∞ has been found to be 5.4 for α-Bi 2 O 3 and 4.9 for Bi 2 O 2 CO 3 . Dielectric spectra of the suspensions have been shown to exhibit no relaxation behavior in the frequency range studied. We have compared the effects of the shape and dielectric permittivity of the particles on the magnitude of the electrorheological effect in them. Using a simple polarization model, we have analyzed the effect of the dielectric characteristics of the filler material on the yield stress τ 0 of the suspensions in electric fields from 0 to 4 kV/mm. Using a combination of rheometric and direct microscopic observations, we have explained the effect of the type of filler on the electrorheological effect: the Bi 2 O 2 CO 3 -based ERFs have higher shear and compressive yield stresses, whereas the α-Bi 2 O 3 -based ERFs have a higher tensile yield stress.