Surface imaging of wet polysulfone (PM30) ultrafiltration membranes with atomic force microscopy indicates a relatively smooth but porous surface in the aqueous environment. The average pore radius was 9.7+/-0.8 nm, and the average porosity was 10%. Therefore, for the case of adsorption of humic acid on the membrane (J. Membr. Sci. 97 (1994) 37-52), we hypothesize here that the transport of humic materials in the membrane can be modeled as one-dimensional diffusion and linear partitioning in a porous medium. An interaction parameter was developed using the model to characterize the strength of interaction between the humic acid and the membrane as a function of pH and calcium concentration. The mass of humic acid adsorbed and the calculated interaction parameter were found to increase with decreasing pH and increasing calcium concentration; hence, the strength of interaction between humic acid and membrane is now simply parameterized. The kinetics of adsorption of humic acid was also modeled, and fitted diffusion time scales showed that the humic acid diffusion coefficient increased with decreasing pH and increasing calcium concentration, which is consistent with a compaction of the humic molecule. These effects were also consistent with a previous study of humic acid diffusion coefficients in activated carbon(J. Colloid Interface Sci. 110 (1) (1986) 149-164).