The kinetics of the transfer of propanoic acid between water and isobutanol have been studied in a Lewis cell. Firstly, the equilibrium was characterized, in order to produce the equilibrium diagram and to evaluate any effects of dissociation and dimerisation. Both the transfer of the solute and the rate of mutual saturation of the two solvents were measured. This method enabled us to evaluate directly the single phase mass transfer coefficients. The adopted procedure represents an improvement of the usual approach, which gives only the overall mass transfer coefficient. The system was found to be affected by interfacial turbulence when the acid is transferred from the water to the organic phase. The corresponding enhancement of mass transfer is located almost completely in the aqueous phase, probably due to its lower viscosity.