It is well known that the rejection of electrolyte mixtures by nanofiltration membranes has a number of particularities. This is suggested to be exploited to obtain additional information on the transport properties and rejection mechanisms of NF membranes from conventional filtration measurements. To avoid the ambiguity related to the possible dependence of membrane properties on the feed composition, it is proposed to use the feeds containing dominant single salts and trace ions whose concentrations are sufficiently low to be not able to influence the membrane properties. It is shown that the rejection of traces is much more sensitive to the rejection mechanisms than that of dominant salts. The shape of dependencies of their rejections on the transmembrane volume flow is qualitatively different for predominantly Donnan and non-Donnan rejection mechanisms and also essentially depends on the sign of membrane charge. That can be used for a qualitative membrane classification. A quantitative interpretation is also possible especially if several traces of different mobilities and charges are used at once.