Protein partitioning kinetics was measured for the semibatch extraction of lysozyme in a laboratory-scale, liquid-liquid spray column. The organic, isooctane phase contained reverse micelles formed from the anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate. For the extraction of protein fi om aqueous to reverse micellar phases, experiments were performed over a range of dispersed-phase flow rates for cases of the organic- or aqueous-phase dispersion. The influence of aqueous-phase pH and ionic strength, which influence electrostatic interactions between protein and reverse micelles, was also investigated. Results were interpreted in terms of a two-film model of mass transfer. The nature of the dispersed phase could significantly influence the partitioning kinetics, while study of the droplet hydrodynamics suggested that stagnant drops were formed regardless of which phase was dispersed. Literature correlations for describing the droplet-formation process and droplet hydrodynamics predicted measured values satisfactorily. Attempts were also made to predict overall mass-transfer coefficients based on existing correlations describing mass transfer during droplet formation, free rise (or fall), and coalescence. Predicted values of K-L were 2-10 times greater than measured values, probably because of large concentrations of surfactant used to formulate the reverse micelle phases. This approach did, however, provide detailed information on the quantity of protein transferred during the successive processes of droplet formation, fi ee rise (or fall) and coalescence.