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The pores of nano-filtration membranes are about 1 nm in size. The retention of loaded and unloaded particles is a defining characteristic of nano-filtration membranes. Experimental filtration studies using carefully chosen molecules can be used to measure the retention of a nano-filtration membrane. For loaded particles, a basic salt solution (NaCl or Na2SO4) is chosen. For unloaded particles, polysaccharides (dextrines) or polyethylene glycols (PEG) of different molecular weights are chosen. A typical nano-filtration membrane has salt retention that is far lower than, for instance, reverse osmosis, while ultra-filtration has salt retention of zero.

Ion selection is a feature of nanofiltration membranes. This is the capacity to differentiate between various ions. Electrostatic repulsion/attraction forces between the elements in the liquid and the (nano-filtration) membrane surface may occur, resulting in a certain level of ion selectivity. This is because a nano-filtration membrane gathers solid loaded groups in its membrane structure. These ions are predicted to diffuse across the membrane based on the sieve effect (pore size 1 nm) and the molecular sizes of chlorides (0.12 nm) and sulphates (0.23 nm). Despite this, the retention for sulfates and chlorides is minimum 90% and 90%, respectively.