An experimental investigation of the dipole interaction in a water‐free glass/oil electrorheological fluid under an alternating electric field is presented in this letter. The values of the dielectric constant &egr; and conductivity &sgr; of glass and oil with respect to the frequency of the external electric field are measured, respectively. Corresponding to the changes in &egr; and &sgr;, the square of the absolute dielectric mismatch parameter |&bgr;| {&bgr;=(&egr;˜p(&ohgr;)−&egr;˜f(&ohgr;)]/[&egr;˜p(&ohgr;)+2&egr;˜f(&ohgr;)], where &egr;˜=&egr;+&sgr;/i&egr;0&ohgr;, and subscriptspandfindicate the glass and oil} decreases as the frequency is increased. After comparing the variation tendency of the |&bgr;|2with the yield stress of the electrorheological (ER) fluid in a measured frequency range we found that they coincide very well. This result confirms that the polarization theory can explain the mechanism of the ER effect not only in a dc electric field but also in that with a wide frequency range. However, the interaction within the multipoles should be considered in an exact calculation. ©1996 American Institute of Physics.