A description is given of a novel technique by means of which experimental studies may be made of the energy dissipation behaviour of materials subjected to biaxial (i.e., combined stress) loading. A specimen in the form a thin cylinder is subjected to simultaneous but separately controlled fluctuating fluid pressures internally, externally and axially. Pressure control is achieved by electro-hydraulic servo-systems and strain response is measured by miniature electrical resistance strain gauges. The apparatus is believed to be unique in its ability to apply biaxial loading with uniform stress distribution and uniform ratio of principal stresses over the full range -1Es2/s1E1 with adequate control and accuracy and absence of mean stress. Hysteresis loops in the two principal directions are recorded. The principles of the apparatus and the special features of its design are discussed. Experimental results are presented. The errors associated with operation of the apparatus correspond to loss factors of the order of 0โข001-0โข002 and hence the apparatus may be considered satisfactory for studies of materials of loss factor 0โข01 and higher.