The mechanism of water extraction into tributyl phosphate—n-hexane solvents has been investigated in a stirred-vessel transfer cell. The effects of stirrer speed, temperature, and the comparison of the TBP-hexane results to those for water transfer into ordinary nonreacting organic solvents strongly suggest that the process is one of simple mass transfer. No effect of complexing, which might have formed the species H20-TBP, was found. The kinetic data (in the form of a single-phase mass transfer coefficient) were all correlated to within ± 10% by the relation where k is the individual mass transfer coefficient, v the kinematic viscosity of the solvent phase, ω the stirrer speed, and Sc the Schmidt number, v/D.