Dispersion-free extraction of solutes from aqueous solutions across a hydrophilic flat-sheet microporous membrane or hydrophobic hollow fiber membrane to an organic phase containing complexing carriers was modeled. The systems of copper(II)-D2EHPA (di(2-ethylhexylphosphoric acid) as well as organic acids such as citric and lactic acids and TOA (tri-n-octylamine) were exemplified. A mass transfer model was proposed taking into account aqueous layer diffusion, interfacial chemical reaction, membrane diffusion, and organic layer diffusion on the basis of a good knowledge of extraction chemistry and the transport properties of the relevant geometry. It was shown that both the calculated extraction rates of Cu(II) and the time profiles of liquid phase concentrations of organic acids were in good agreement with the measured ones (average standard error, 11%). The rate-controlling mechanisms of such dispersion-free membrane extraction processes were finally identified and discussed. (C)2000 Elsevier Science B.V. All rights reserved.