This is a study of the nanofiltration of various charged amino acids and peptides, in solutions of increasing complexity, through an inorganic membrane. Whatever the solution studied, charge effects, repulsion of colons and attraction of the counterions, more than size effects, prevail in the behavior of the solute. The extent of the transmission of a given solute, and hence the selectivity of the separation, were strongly affected by solute environment, mainly because of competitive solute/solute and solute/membrane interactions. However, despite the complexity, the experimental data were always in accordance with the following principles : (i) whatever solution, the electric neutrality was always recovered and (ii) the number of charges, rather than global net charge, should be considered in order to account for amino acid and peptide transmissions. On the basis of these principles, a general mechanism for amino acid and peptide separation in nanofiltration is proposed.