Permeation of lanthanides (Ln) between aqueous nitrate solutions of different acidity through supported kerosene membrane containing di-(2-ethylhexyl)phosphoric acid (DEHPA) as a carrier has been studied. Mass transfer of Ln cations has been determined in terms of permeability coefficients (P) by on-line FIA measurement of metal concentrations in strip solution. Acidity of feed solution ranged from pH 1.2 to 4.2 (ionic strength 0.1 mol l(-1) (HNO3, NaNO3)) while that of strip solution was kept constant at 0.1 mol l(-1) HNO3. Carrier concentration has been changed in the range from 10 to 10(3) mmol l(-1). Both, the P versus feed pH plots measured at low and at high DEHPA concentrations as well as P versus DEHPA concentration plots exhibited maxima. Permeability coefficients were decreasing when advancing in the lanthanide series and significant differences in behaviour of individual lanthanides has been observed. The conditions of separation of light and heavy lanthanides and mutual separation of light lanthanides have been suggested. Based on these and previous results, composition of transported species, presence of maxima on measured curves, and formation of gel-like substance in feed solution and on membrane-feed solution interface are discussed.