An improved instrument for the precise measurement of interfacial tension between two fluid phases by means of a drop of the less dense fluid surrounded by the more dense fluid, both fluids spinning together in gyrostatic equilibrium. Elements of the invention eliminate departures from gyrostatic equilibrium which went unrecognized in the prior art. These elements are based on hydrodynamic principles unknown in the prior art. The instrument is characterized by the combined use of air bearings and sample-holder mountings which reduce vibrations and eccentricities, eliminate temperature gradients within the sample, and assure that interfering hydrodynamic flows are suppressed. Other improvements include biprecision sample tube, full visual access to the spinning drop, leveling provisions, timing belt transmission, avoidance of bearing heating and wear problems, and convenient control of sample temperature.