Diffusion experiments aided by high performance size exclusion chromatography were carried out to describe the size exclusion properties of microcapsules prepared from cellulose sulphate and poly[diallyldimethylammonium chloride] (PDADMAC). The membrane cut-off was characterised by the dependence of the distribution ratio (luminal concentration/external concentration) of dextran molecules on their Stokes＇ radius (1-9 nm) after diffusion periods of 72 and 240 h. When capsules were prepared from standard cellulose sulphate and different polydisperse PDADMAC preparations with high mass portions of size fractions > 10 kDa, the cut-off range was narrow with an upper size limit (USL) of permeation close to 2 nm and independent of the residence time in the PDADMAC bath or the presence of salt (155 mM NaCl) in the reacting polymer solutions, Capsules prepared with PDADMAC rich in size fractions with lower molecular weight (<3 kDa) showed higher values of the USL. The cut-off was dependent on the residence time and salt concentration when capsules were produced using polydisperse PDADMAC with high mass portions of small molecules (<0.6 kDa). Prolonged residence of such capsules in a reaction bath containing 155 mM NaCl resulted in a strong increase of the USL.