Filmand continuous-phase mass transfers in liquid-liquid systems were studied using a cocurrent laminar wetted-wall column of a modified form. The film-phase mass-transfer coefficients, obtained by dissolution of MIBK, 1-butanol, and cyclohexanol in water, were in good agreement with the Beek-Bakker model, demonstrating the advantage of this column because of its negligible end effect. The continuous-phase mass-transfer coefficients were measured by the transfer of I2 dissolved in continuous phases of ethylhexyl alcohol, toluene, and n-hexane into the interface, where I2 disappeared by an instantaneous irreversible reaction with Na2S203 in the aqueous film phase. These coefficients were in satisfactory agreement with the penetration theory when the driving force of solute I2 was evaluated by the method proposed in this paper.In all fundamental studies for liquid-liquid mass transfer, it is desirable to use an experimental apparatus in which the interfacial area and hydrodynamics are well defined. However, few such apparatus have been available.As one kind of suitable apparatus, cocurrent laminar wetted-wall columns have been used by Maroudas and Sawistowski (1964) and Bakker et al. (1967) for mass