A method using calibrated drops, freely falling through a stationary continuous phase, is commonly used to study the kinetics of heat or mass transfer. This technique has been applied to uranium isotopic transfer between free ions dissolved in an aqueous phase and the corresponding complexes of an organic phase. In this report, results are presented for experiments in which an aqueous hydrochloric acid solution of U(IV) of naturally occurring isotope composition and an organophosphorous extractant, diluted in aromatics, containing a U(IV) complex at 235U atom fraction of 0.5% were used to determine isotopic mixing kinetics. The aqueous phase was fed as calibrated drops through the stationary organic phase at a velocity depending on the drop size and the physical characteristics of the contacted phases. The 235U concentration of the effluent stream was determined as a function of the drop residence time. An isotopic transfer coefficient kA (cm/sec) was calculated. Its variation was investigated as a function of drop diameter between 0.07 to 0.4 cm. The plot of kA vs diameter can be divided in two regions where kA is practically constant. In the region of small drops, the activation energy of kA was measured. The low value, 6 kcal/mole, indicates diffusional, rather than chemical transfer resistance. In the diffusion control region the isotopic exchange kinetics are controlled by the physical characteristics of the phases. A model which was developed from this studyis shown to be useful in optimizing the operation of the process.