Electrorheological fluids can exhibit highly nonlinear behavior for large electric‐field strengths and strains. Oscillatory rheometry of these activated suspensions can, in some cases, result in the dynamics of the rheometer contributing to the measured shear stress within the ER material. Some experimental results are presented for an activated electrorheological fluid for a range of experimental conditions. These results show how the material behaves under an increasing electric field, for certain levels of strain and oscillatory frequencies. The results also show the effects of the inertia of the viscometer on the observed characteristic behavior of the material, and a method of removing this component from the data is discussed. A three element mechanical analogy of the ER material is presented, which together with the introduction of an inertial term models well the behavior seen in the experimental results. By taking into account some of the influences of the experimental apparatus, the proposed model is used to put forward some hypotheses for the characteristic physical response of an activated ER material.