Electrorheological (er) fluids consist of micron-sized, semi-conducting particles suspended in an electrically insulated continuous phase such as hydrocarbon or silicone oil. such fluids exhibit a significant and reversible increase in their resistance to flow when subjected to an electric field. conventionally, it is the shear properties of er fluids which have been evaluated and exploited in engineering devices. in this thesis the author presents what is believed to be the first comprehensive evaluation of the properties of er fluids subjected to tensile and compressive loads. this is justified by the failure of the shear mode of operation to meet industrial specifications. because of the fundamentally different mechanical loading mechanisms required to place er fluids in shear ortension/compression the first series of experiments describes a microscopic study aimed at elucidating the structure of particle chains under the different forms of loading. in the usual shear configuration the inter-electrode gap and thus the electric field is normally constant for a constant applied voltage. however, in tension/compression, the inter-electrode gap is constantly varying and a feedback control circuit is required to maintain a constant electric field. using such a circuit a study is presented to compare the behaviour of er fluids in tension and compression under both constant voltage and constant field conditions. a new method for visualisation has been developed and, depending upon the form of electrical excitation, two distinct models of behaviour have been identified. the main body of the thesis is concerned with the mechanical properties of an er fluid when subjected to tensile and compressive loading. the principal structural property effected by the electric field is the yield stress and thus stress-strain characteristics are examined under both direct current and alternating current excitation. these characteristics are compared with those obtained with the fluid in shear mode and thus the advantages in terms of mechanical strength of the tensile/compressive mode are demonstrated conclusively. most of the tests were performed using er fluids developed at liverpool but the performance of commercially available er fluids has also been examined. finaliy, because one of the major obstacles in the path of industrial exploitation is the temperature dependence of er fluids, apparatus was developed for testing fluids at elevated temperatures when subjected to tensile and compressive loads. the results show that the improved stress levels obtained under tension and compression are maintained at temperature of up to 90°c. for example, prolonged use of a water-activated er fluid (eg. silicone oil and starch) at temperatures of 90°c showed stress levels of approximately 14 kpa (in tension) and over 18 kpa (in compression). this demonstrates conclusively that a simple er fluid can function in a critical condition, at an elevated temperature. furthermore, the prediction of the stress over the cycle of operation and hence at a given working temperature is now well established.