A basic property of the curvature of pendant drops and computer splines have been used to develop a simple and highly reliable method for the determination of interfacial tension of fluid-fluid interfaces using a drop shape technique. Solutions to the Laplace-Young equation for pendant drops predict a linearly varying curvature as a function of elevation. Plotting the mean curvature of the pendant drop versus elevation renders a straight line with slope a = -DELTArho g/sigma, where DELTArho is the difference in densities of the two fluids, g is the acceleration of gravity, and sigma is the interfacial tension. Numerical differentiation of experimental data, however, is nearly impossible when any level of noise is present. Computer-generated spline functions were used to represent the experimental data to a prescribed degree of smoothness. In turn, derivatives of the spline functions provide an accurate and reliable way to determine the curvature of the drop image. The method has been tested with the computer-generated image of a water drop and with experimental data for five hydrocarbons as well as decyl alcohol.