Transport of aniline through a polyethylene terephthalate (PETP) track-etched membrane (pore size 0.03 mu m, thickness 10 mu m) without an external electric field has been studied. Diffusion is accompanied by the accumulation of aniline in the pore solution (concentrations of the penetrant in the pores are 102 times as large as in the feed). Aniline is likely to be protonated within the membrane even at pH of the external solution exceeding the pK(a) of aniline salt. Transport only occurs through a deprotonated membrane (pH > 6). Inorganic electrolytes accelerate or retard the diffusion of aniline, depending on their nature and concentration (from a 2 mol/L NaCl solution aniline is transported even against its concentration gradient). Transport of aniline correlates with the difference in mobilities of cation and anion of the salt (LiCl and CsCl are exceptions), which implies the diffusion potential created by salt diffusion to be the driving force of aniline transport. Most of the said features of aniline diffusion resemble the diffusion behavior of cations and are explainable by the existence of aniline in the membrane in a cationic form. Experimental estimate of pH in the pores agrees with this suggestion. The regularities observed for aniline may apply to other ionizable penetrants. (C) 2009 Elsevier B.V. All rights reserved.