Experimental studies have been made on the sorption and diffusion of selected carboxylic acids-acetic acid (AA), lactic acid (LA), oxalic acid (OA) and tartaric acid (TA)-in a strongly basic polymeric membrane Neosepta-AMH at 25 degrees C. The sorption experiments have revealed that the partition coefficients (Psi), which generally decrease with an increasing acid concentration in the external solution, correspond to the following series: Psi(TA) >Psi(OA) > Psi(AA) >Psi(LA). The measurements of diffusivity have shown that diffusivities in the membrane at infinite dilution are approximately two orders of magnitude lower than those in aqueous solutions. Diffusivity (D) of acid in the membrane at infinite dilution follows the series D-TA < D-LA < D-AA < D-OA and it can be correlated with that in aqueous solution; this correlation is given graphically. Moreover, the concentration dependences of diffusivity of selected carboxylic acids have been determined. Except for tartaric acid, all the remaining acids have exhibited an increase in diffusivity with an increasing acid concentration in the membrane.