A dynamic interfacial tensiometer described measures the tension of stressed liquid-liquid interfaces. In this accurate and controlled method, the time-dependent pressure difference across the curved interface of a spherical drop, immersed in a second immiscible phase, is measured as a function of time. Dynamic experiments are conducted by varying the drop size, thereby stressing the interface. The technique is eminently suited for the measurement of static and dynamic interfacial tensions, the study of the diffusion, adsorption, and desorption kinetics of surfactants, and the rheological behavior of liquid films. These capabilities ave demonstrated for n-dodecane contacted with an aqueous solution of a nonionic surfactant. To explain the phenomena, diffusion models are developed that compare well with the experimental data. The noninvasiveness and versatility make this instrument a useful tool.