An engine mount includes a main working chamber and an expansion chamber. Orifice passages in which electrodes are disposed, fluidly connect the main and expansion chambers. An auxiliary chamber which is hermetically separated from the main working chamber by a flexible partition is in part defined by a diaphragm the external surface of which is exposed to atmospheric pressure. The auxiliary chamber contains an ERF which develops a higher viscosity than the ERF which fills the main and expansion chambers. When a voltage is impressed on electrodes in the auxiliary chamber the viscosity of the ERF increases and the flexure of the flexible elastomeric partition is prevented.