A one-dimensional fine-porous membrane model has been exploited to determine the reflection coefficient, sigma, and the internal solute mass transport coefficient, k(m), for the nanofiltration of simple salts and dyes by two weak-acid membranes, HR and LR, differing in sigma for a NaNO3 solution. The rejection, r, was strongly dependent on the crossflow velocity, u, and the flux, J, in these experiments. The dependence of the external solute mass transport coefficient; k(s) on u was also determined. The dominant parameter in determining the limiting, low J and high u, rejection for the systems is sigma. It was influenced primarily by the charge of the colon for the simple sodium salts, NaNO3 and Na2SO4. The k(m) remained constant within experimental error, for NaNO3 and Na2SO4 using membrane HR and for the two dyes NaR4 and Na(2)R1 using membrane LR even though the charge on the colons and their formula weights differed in each comparison. Transitions from regime I, i.e., decreasing r with increasing J at constant u, to regime II, i.e., increasing r with increasing J at constant u, behavior were observed at low u for a trivalent dye using membrane LR.