The extraction of organic acids such as citric and lactic acids from aqueous solutions through a hydrophobic microporous flat-sheet membrane to an organic phase containing tri-n-octylamine (TOA) was examined experimentally and theoretically. A mass transfer model was proposed, considering aqueous layer diffusion, interfacial chemical reaction, and membrane and organic layer diffusions. It was shown that the time profiles of liquid-phase concentrations of the acids were in reasonable agreement with the measured ones (average standard error, 8%), especially well at low feed acid and/or TOA concentrations. Based on a good knowledge of kinetics of liquid–liquid extraction (LLE), the present processes were found to be governed by mixed interfacial reaction and membrane diffusion.