The general wear performance and boundary lubrication properties of a silicone-oil-based electro-rheological (ER) fluid are presented. The tribological characteristics are derived from the results of an extensive series of tests on a two-disc machine. By comparison with a commercial lubricating oil working under the same conditions it is concluded that the ER fluid tested is likely to cause severe wear problems in some engineering situations. This is so even though the active particulate matter of the fluid mixture appears to act as a solid lubricant which enhances the performance of the base liquid phase.