A type of water-free electrorheological (ER) material-complex strontium titanate (STO) was synthesized by means of modified sol–gel. The ER behavior of the suspensions of STO particles in silicone oil with a 37% volume fraction was investigated systematically under both dc and ac electric fields. It is found that this ER fluid has many advantages, such as long-term stability against sedimentation, strong ER effect at low electric field, and a wide operating temperature range. The frequency dependence of ER behavior was also studied and can be partly explained on the basis of dielectric measurement. © 1998 American Institute of Physics.