Nanofiltration (NF) membrane technology shows interesting potentials for separating organic components on the basis of solute charge and size in the range of 300-1000 g mol(-1). Separation properties of two inorganic NF membranes were studied with a set of 10 small peptides (molecular mass range: 300-900 g mol(-1); 3 < pI < 10) contained in a well-characterized tryptic β casein hydrolysate. Peptides transmission strongly depended on ionic interactions in the system. Physicochemical conditions such as ionic strength and especially pH were crucial to the separation, because the membrane and peptides showed amphoteric properties. Thus, the three categories of peptides (acid, basic, neutral) were separated according to their pI because of presumed concentration gradients of charged peptides at the membrane: positive for basic peptides and negative for acid peptides. At optimum pH 8 this led to high transmissions of basic peptides (even over 100%), intermediate transmissions for neutral peptides, and low transmissions for acid peptides. The addition of multicharged cationic and anionic species in the hydrolysate induced a markedly enhanced selectivity when the polyelectrolyte was a membrane coion and a complete reversion of selectivity when it was a membrane counterion.