Thin membranes of an aliphatic polyamide (nylon-6,6) and an aromatic polyamide (polyxylyleneadipamide) (PXAP) were prepared, and their distribution (K) and overall diffusion (D) coefficients of sodium chloride were measured with the unsteady-state and steady-state dialysis method. The overall diffusion coefficients at a zero concentration [D-(0)] of sodium chloride for nylon-6,6 and PXAP were 1.3-0.8 mum(2)/s (from 2 min of interfacial polymerization to 4 min) and 0.078, respectively. D-(0) for PXAP was about 3 times greater than that of a cellulose acetate (CA) membrane (0.024 mum(2)/s). The K values for nylon-6,6 and PXAP were 0.7-0.5 from 2 to 4 min and 0.05, respectively. K for PXAP was almost the same as K for CA (0.06). A two-part (dense and porous) model of the membrane structure was applied to obtain D-d (the diffusion coefficient in the dense part of the membrane) and D-P (the diffusion coefficient in the porous part of the membrane) for CA, PXAP, and nylon-6,6 thin membranes. The values of D-d were almost the same for both nylon-6,6 and PXAP (0.05-0.061 mum(2)/s) and about 10 times greater than the value for the CA membrane (5.6 x 10(-3) mum(2)/s). D-P for PXAP was almost the same as D-P for CA. However, D-P for the nylon-6,6 membrane was 10-16 times greater than D-P for the PXAP membrane.