The electrorheological response of hydrated poly(methacrylate) particles in a chlorinated hydrocarbon has been characterized as a function of field strength, shear rate, volume fraction and temperature. Viscosities of electrified suspensions are increased up to three orders of magnitude over the unelectrified suspension at low shear rates but decrease to the value of the unelectrified suspension at high shear rates. The observed response has been analysed by considering the fundamental forces acting on the particles and it is found that, at a given volume fraction, all of the dependencies of relative viscosity on shear rate, field strength and temperature can be collapsed into a single function of the Mason number [η_c/2ε₀ε_c(βE)²]. The results have been further analysed in terms of a Bingham Plastic constitutive equation from which a critical Mason number naturally arises characterizing the transition from polarization to hydrodynamic control of suspension structure.