Surface modification of nanofiltration cellulose acetate (CA) membranes was carried out by alternating layer-by-layer deposition of acidic chitosan (CID) and sodium alginate (ALG) as the cationic and anionic polyelectrolyte, respectively. The supporting CA membranes were obtained by a phase separation process from acetone/formamide (2/1 wt/wt) casting solutions using a 20 wt% polymer concentration. The pore size of the CA substrates monitored by the annealing temperature was determined from the retention values of rigid neutral solutes using the computer simulation program NanoFlux. The influence of the ultrathin multilayered polyelectrolyte surface structure on the permeability performances (water flux and ion rejection) was studied for a series of composite membranes obtained by varying the total number (up to 35) of the adsorbed polyelectrolyte layer pairs. SEM and permeation measurements showed that the modification mainly took place on the substrate surface so that the hydraulic permeability was only little affected by the polyelectrolyte multilayers. The permeation rate of dilute to moderate concentrated NaCl and MgSO4 solutions was found to be higher than that of pure water for some composite membranes. By comparison with the bare support, the rejection of monovalent salt decreased by increasing the number of adsorbed ALG/CHI layer pairs whereas that of divalent salt remained constant.