One of the earliest transport models for membranes was given by Kimura and Sourirajan [l] in 1967. Their two-parameter model, termed the preferential sorption-capillary flow model, characterized the performance of membranes for many solution systems. Ohya et al. [2,3] subsequently adopted this model to solve for the separation performance of commercial RO permeators. However, the resulting system of solution consists of eight equations, two of which require numerical integration that must be solved simultaneously in order to obtain the non-dimensional transport characteristics, X and 8. In this paper a simplification is introduced in the integral expression leading to an analytical solution. For conditions typical of seawater desalination operations, it is shown that the discrepancy introduced in the integrals as a result of the simplification is much less than 1 .O % , while for the concentration of fruit juice, the resulting error is about 2.0%. SYMBOLS A -pure water permeability coefficient, cm2 s/g Ci -(&i>I(~I>, d imensionless mole fraction ratio c -(~~i)/(~~l), dimensionless mole fraction ratio at initial conditions CP -average product mole fraction ratio DAM -solute diffusivity in membrane, cm2/s f -fractional solute rejection