The dynamic properties of two kinds of aluminosilicate-based electrorheological (ER) fluids were investigated under on-state and off-state electric fields. The dynamic responsive (frequency sweep spectrum and strain sweep spectrum) differences between these two ER suspensions were found under the off-state electric field, whereas similarities were observed once an external electric held was applied. Their de conductances under an oscillatory mechanical held and their mechanical strengths under an oscillatory electric held were also examined. The periodically varied dc current in an oscillatory mechanical field and the decreased shear stress in an ac field were both found in these ER suspensions. The different in situ microstructures of these two ER fluids suggested on the basis of the percolation model, as web as their different response times measured experimentally, were presented to understand the experimental observations. Our results have a direct implication concerning the practical application of ER fluids.