Liquid membrane transport (pertraction) of propionic (PA) and/or acetic (AA) acid has been studied in a multimembrane hybrid system (MHS): feedparallel toAEM\BLM\AEMparallel to stripping solution. The MHS was made up of strongly basic anion-exchange membrane (AEM) and a bulk liquid membrane (BLM) composed of hexane and tri-n-octylphosphinic oxide (TOPO) or tri-n-butylphosphate (TBP). Transport rates, and carrier-dependent facilitation factors (F) were determined as dependent on the feed and carrier concentration. It was found that TBP facilitates the pertraction of AA (F approximate to 8) and does not influence the pertraction of PA, which predominantly permeates the BLM according to the solution-diffusion mechanism. In the case of TOPO, the negative effect of transport freezing characterized by F < 1 was observed. According to experimental findings, the following order of fluxes has been established and verified in competitive transport experiments: J(AA)(TBP) > J(PA)(hexane) > J(PA)(TBP) > J(AA)(TOPO) > J(AA)(hexane) > J(PA)(TOPO). The facilitation factors change typically in the sequence: (AA-TBP, F > 1) > (AA-TOPO, F > 1) > (PA-TBP, F less than or equal to 1) > (PA-TOPO, F < 1). (C) 2002 Elsevier Science B.V. All rights reserved.