An electrorheological fluid of a particulate phase and a continuous phase of a hydrophobic liquid medium, a certain dispersed particulate phase, and a low molecular weight polar material exhibits a broad effective temperature range. The dispersed particulate phase comprises a polar solid material which is capable of exhibiting substantial electrorheological activity only in the presence of a low molecular weight polar material, and an organic semiconductor. The weight ratio of the polar solid material to the organic semiconductor is at least about 2:1.