To reduce costs, engineers in the industry wish to substitute laboratory-scale experiments for pilot-scale investigations. Thus mass-transfer experiments with single drops seem to be a first step toward the design of liquid- liquid extraction columns, along with other experiments needed for example, for the estimation of coalescence parameters. In this article it is shown that it is possible to describe nonstationary mass transfer between single drops and a continuous phase with a single parameter, the instability constant C-IP. The instability constant is independent of both drop diameter and contact time between drop and continuous phase. Thus in principle a single laboratory experiment allows the determination of C-IP for a given combination of components. When mass transfer in an extraction column is calculated using the estimated value of C-IP, an increase in mass transfer due to the concentration gradient in the continuous phase of the column has to be accounted for appropriately.