An electro-rheological fluid which comprises a solid particulate substance contained in a hydrophobic vehicle which is liquid at atmospheric pressure at least at temperatures below 50.degree. C. and which comprises a compound of the formula:(X).sub.n --Ar[Q--Z].sub.pwherein:Ar represents an aromatic nucleus;Q represents an oxygen or a sulphur atom, or a group of the formula CY.sub.1 Y.sub.2, SO, SO.sub.2, SiF.sub.2, --OSi(Y.sub.1 Y.sub.2)O-- in which Y.sub.1 and Y.sub.2, which may be the same or different, each represents a halogen or a fluorine atom or an alkyl group;X represents a halogen atom, or a nitro group, a thio(substituted or unsubstituted hydrocarbyl) group or a substituted or unsubstituted hydrocarbyl group;Z represents a substituted or unsubstituted aliphatic or alicyclic group; andn and p, which may be the same or different, each represent a number of at least 1, (n+p) not being greater than the total number of substituted sites on the aromatic nucleus, with the proviso that, where n is greater than 1, not all the n X groups need be the same and that the, or at least one of the, X group(s) represents a halogen atom; and that, where p is greater than 1, not all the pQ groups nor the pZ groups need to be the same.