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This paper describes an instrument to measure. the interfacial tension (IFI) of aqueous surfactant solutions and crude oil. The method involves injection of a drop of fluid (such as crude oil) into a second immiscible phase to determine the IFT between the two phases. The instrument is composed of an AT-class computer, optical cell, illumination, video camera and lens, video frame digitizer board, monitor, and software. The camera displays an image of the pendant drop on the monitor, which is then processed by the frame digitizer board and non-proprietary software to determine the IFT. Several binary and ternary phase systems were taken from the literature and used to measure the precision and accuracy of the instrument in determining IFTs. The instrument has been used to determine the IFTs for several bacterial supernatants and unfractionated acid precipitates of microbial cultures containing biosurfactants against medium to heavy crude oils. These experiments demonstrate that the use of automated video imaging of pendant drops is a simple and fast method to reliably determine interfacial tension between two immiscible liquid phases, or between a gas and a liquid phase. Introduction It has been estimated that approximately 60% of all oil discovered will remain trapped after current oil recovery technologies have been employed. Waterflood recovery is the most economic and widely applied secondary recovery technique. However, waterflooding does not remove all the remaining oil in the reservoir due to incomplete reservoir flooding and to Viscous and capillary forces. The unproduced oil remaining after waterflooding includes immobile oil left in place. The immobile oil is trapped in pore structures by viscous and capillary forces, and cannot be removed by waterflooding. Ultra-low IFTs (about 10 mN/m) allow the oil trapped by capillary forces to become mobile. Lowering the IFT by use of surfactants decreases the pressure required to force a nonwetting phase (oil) through small capillaries and pore constrictions. Micellar (or surfactant) flooding is of particular interest at the Idaho National Engineering Laboratory (INEL) due to research on microbial enhanced oil recovery (MEOR) being conducted there. Measurement of IFTs has been accomplished by a variety of methods, including pendant drop, sessile drop, and spinning drop methods. These methods place a drop of oil into a surfactant solution, and then measure the shape of the drop formed. Current developments in personal computers coupled with the ability to digitize video images enables the researcher to automate the process, and provides an improved process to determine the IFT. Video digital methods have been applied to the measurement of the IFTs of bacterial biosurfactants. A simple, fast, automated system for measuring interfacial tensions using the pendant drop method has been developed at the INEL. THEORY OF IFT DETERMINATION BY PENDANT DROPS Capillary forces between crude oil and formation water determine the pressures needed to displace the oil from formation pores, which is given by the Young-Laplace equation. (1) P. 513^