Certain polyelectrolytes, when rigorously dried and dispersed into a nonconducting liquid, produce dispersions which may be electrorheologically active at 23 and 100 °C, at 100 °C but not at 23 °C, or not active at either temperature. Further, fluids apparently ER inactive at room temperature but active at elevated temperatures have not previously been reported. In this paper, three primary chemical characteristics of the polyelectrolyte inclusions have been examined to help in understanding this behavior and on a broader scale to aid in better understanding of fundamental mechanisms responsible for ER effectiveness. These include dissociation constants of the polyelectrolytes, counter ion type, and matrix resin type of the macroions. The results show a strong relationship between ER behavior andpKa, whereas no clear dependencies seem to exist with the other parameters. This additionally supports the hypothesis that ER activity is associated with mobile charges bound to the particulate phase. Additionally, the fact that some of these materials work much better at 100 °C than at room temperature further supports the contention that the behavior is not water mediated and another mechanism is involved, i.e., a mechanism intrinsic to the chemistry of the particulate phase.